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Acids, Bases and Salts PYQs – Chemistry, SSC & Railway

172 Acids, Bases and Salts (Chemistry) questions asked in recent SSC and Railway exams, newest first, with exam, date & shift, answers and solutions – page 2 of 2.

📚 172 questionsSSC 9 · Railway 163
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RRB NTPC GL CBT-1 2024-25 – 5 June 2025 Shift 1 · Nature of salts (acidic, basic, neutral)

Q101. Which of the following solutions is slightly acidic due to hydrolysis?

  1. (a) NH₄CH₃COO
  2. (b) Na₂CO₃
  3. (c) CH₃COONa
  4. (d) NH₄Cl
Answer & solution

Answer: (d) NH₄Cl

RRB Technician Grade III 2024-25 – 30 December 2024 Shift 1 · Water of crystallisation

Q102. How many water molecules are present in plaster of Paris?

  1. (a) Three molecules of water
  2. (b) Half molecules of water
  3. (c) Zero molecules of water
  4. (d) One molecule of water
Answer & solution

Answer: (b) Half molecules of water

Correct Answer: Option 2 — Half molecules of water
Plaster of Paris is calcium sulphate hemihydrate, whose formula is CaSO₄·½H₂O (often written as 2CaSO₄·H₂O). The prefix hemi means half, so one formula unit of the salt holds only half a molecule of water of crystallisation — put the other way round, two formula units share one water molecule.
It is manufactured by heating gypsum (CaSO₄·2H₂O) to about 373 K (100 °C). At this temperature gypsum loses three-fourths of its water of crystallisation and changes into plaster of Paris, a white powder.
When plaster of Paris is mixed with water it takes the water back and sets into a hard solid mass of gypsum again. This setting property is why it is used for plaster casts of fractured bones, for statues, toys, chalk and false ceilings, and as a fire-proofing material.
⚗️ The Reaction
CaSO₄·2H₂O → (heating, 373 K) CaSO₄·½H₂O + 1½H₂O
• Gypsum gives up part of its water of crystallisation to become plaster of Paris.
• Setting is simply the reverse change: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
• Because it absorbs moisture from the air, it must be kept in a moisture-proof container.
❌ Why the other options are incorrect
✗ Three molecules of water — no hydrate of calcium sulphate carries three water molecules; gypsum, the most hydrated form, carries only two (CaSO₄·2H₂O).
✗ Zero molecules of water — water-free CaSO₄ is anhydrite, and the form made above 473 K is dead burnt plaster, which has lost its setting property.
✗ One molecule of water — that would be the monohydrate CaSO₄·H₂O. The formula 2CaSO₄·H₂O confuses aspirants: one water is shared by two units, so each unit gets half.
📌 SSC & Railway Exam Facts
• Plaster of Paris — CaSO₄·½H₂O; gypsum — CaSO₄·2H₂O.
• The name comes from the huge gypsum deposits of Montmartre, Paris (France).
• Gypsum is heated to 373 K (100 °C); above 473 K it becomes dead burnt plaster.
• Chemical name of plaster of Paris: calcium sulphate hemihydrate.
• Uses: plaster casts for fractures, statues and toys, chalk, false ceilings, fire-proofing.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 1 · Neutralisation reaction

Q103. Which of the options represents the correct and balanced chemical equation for the reaction between a metallic oxide and an acid?
CuO + HCl → ______ + ______

  1. (a) CuO + HCl → CuCl + H₂
  2. (b) CuO + HCl → CuOH + HCl
  3. (c) CuO + 2HCl → CuCl₂ + O₂
  4. (d) CuO + 2HCl → CuCl₂ + H₂O
Answer & solution

Answer: (d) CuO + 2HCl → CuCl₂ + H₂O

Correct Answer: Option 4 — CuO + 2HCl → CuCl₂ + H₂O
Metal oxides are basic in nature, so when a metal oxide meets an acid the two neutralise each other and give a salt and water. The general rule is Metal oxide + Acid → Salt + Water, which is why such a reaction is also called a neutralisation and belongs to the double displacement family.
Here copper(II) oxide (CuO), a black solid, reacts with hydrochloric acid to give copper(II) chloride (CuCl₂) and water. Copper in CuO is in the +2 state, so it needs two chloride ions — hence CuCl₂ — and therefore two molecules of HCl. The black CuO dissolves and the solution turns blue-green.
⚗️ The Reaction
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• Count the atoms — Cu 1 = 1, O 1 = 1, H 2 = 2, Cl 2 = 2: the equation is balanced.
• O²⁻ of the oxide picks up the 2H⁺ of the acid to make H₂O; Cu²⁺ pairs with 2Cl⁻.
• No hydrogen gas is released — a metal *oxide* is not a metal, so this is not a displacement reaction.
❌ Why the other options are incorrect
✗ CuO + HCl → CuCl + H₂ — wrong on two counts: copper here is +2, so CuCl (Cu in +1) is the wrong salt, and H₂ gas is never produced when an oxide meets an acid. It is also unbalanced (O appears only on the left).
✗ CuO + HCl → CuOH + HCl — HCl cannot appear unchanged on both sides, and CuOH is not the product of an acid reaction; nothing is really reacting here.
✗ CuO + 2HCl → CuCl₂ + O₂ — the formula demands H₂O, not O₂: on the left there are 2 H and 1 O, so the hydrogen would vanish and the oxygen atoms would not balance.
📌 SSC & Railway Exam Facts
• Metal oxide + Acid → Salt + Water (metal oxides are basic).
• Non-metal oxide + Base → Salt + Water (non-metal oxides are acidic), e.g. CO₂ + 2NaOH → Na₂CO₃ + H₂O.
• CuO is black; CuCl₂ solution is blue-green; CuSO₄·5H₂O is blue vitriol.
• Metal + Acid → Salt + Hydrogen gas — that is where H₂ comes from, e.g. Zn + 2HCl → ZnCl₂ + H₂.
• A balanced equation obeys the law of conservation of mass (Lavoisier).
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 1 · Neutralisation reaction

Q104. Which of the following is produced when acid reacts with a base?

  1. (a) Salt and water
  2. (b) Only water
  3. (c) Salt and hydrogen ion
  4. (d) Salt and hydrogen gas
Answer & solution

Answer: (a) Salt and water

Correct Answer: Option 1 — Salt and water
The reaction between an acid and a base is called a neutralisation reaction, and it always follows the pattern Acid + Base → Salt + Water.
An acid supplies H⁺ (hydrogen) ions in water, a base supplies OH⁻ (hydroxide) ions. In the reaction these two combine to form H₂O, and the remaining positive ion of the base joins the remaining negative ion of the acid to give the salt. Because both the acidic and the basic character are destroyed, the mixture moves towards pH 7 — the two have *neutralised* each other. It is also a double displacement reaction and is normally exothermic.
⚗️ The Reaction
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• The essential step is H⁺ + OH⁻ → H₂O.
• Na⁺ (from the base) pairs with Cl⁻ (from the acid) to give the salt NaCl.
• Heat is given out, so neutralisation is an exothermic reaction.
❌ Why the other options are incorrect
✗ Only water — the metal ion of the base and the acid radical do not disappear; they must combine and appear as a salt, so water alone is an incomplete answer.
✗ Salt and hydrogen ion — the H⁺ ion is used up in forming water; if free H⁺ were left over the solution would still be acidic and no neutralisation would have occurred.
✗ Salt and hydrogen gas — H₂ gas is released when an acid reacts with a metal, not with a base: Zn + 2HCl → ZnCl₂ + H₂↑. This is the most common confusion in this question.
📌 SSC & Railway Exam Facts
• Acid + Base → Salt + Water (neutralisation, a double displacement reaction).
• Acid + Metal → Salt + H₂ gas; the gas burns with a pop sound.
• Acid + Metal carbonate/bicarbonate → Salt + Water + CO₂; CO₂ turns lime water milky.
• pH 7 is neutral, below 7 acidic, above 7 basic; the pH scale was given by Sørensen.
• Everyday neutralisation: antacid for acidity, baking soda on a bee sting, lime on acidic soil.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · Properties of bases and alkalis

Q105. When sodium hydroxide reacts with water, which of the following products is/are formed?

  1. (a) Both Na⁺(aq) and Cl₂
  2. (b) Only Na⁺(aq) ions
  3. (c) Both Na⁺(aq) and OH⁻(aq) ions
  4. (d) Only OH⁻(aq) ions
Answer & solution

Answer: (c) Both Na⁺(aq) and OH⁻(aq) ions

Correct Answer: Option 3 — Both Na⁺(aq) and OH⁻(aq) ions
Sodium hydroxide (NaOH), commonly called caustic soda, is a strong base. When it is put into water it dissolves completely and dissociates (ionises) into a sodium ion, Na⁺, and a hydroxide ion, OH⁻. Each ion is surrounded by water molecules, which is shown by writing (aq) after it.
A base that is soluble in water and furnishes OH⁻ ions is called an alkali. It is this large supply of OH⁻(aq) that makes the solution bitter in taste, soapy to touch, gives it a pH greater than 7, and turns red litmus blue.
Notice that no chlorine can appear here — the formula NaOH contains only Na, O and H. Chlorine is released only from a chlorine-bearing compound such as NaCl.
⚗️ The Reaction
NaOH(s) + water → Na⁺(aq) + OH⁻(aq)
• The ionic bond between Na⁺ and OH⁻ is pulled apart by polar water molecules.
• Dissolving NaOH in water is strongly exothermic, so a great deal of heat is released.
❌ Why the other options are incorrect
✗ Both Na⁺(aq) and Cl₂ — this is the common mistake of confusing NaOH with NaCl; NaOH has no chlorine atom, so Cl₂ can never form.
✗ Only Na⁺(aq) ions — a solution must remain electrically neutral, so the negative OH⁻ ions are produced along with Na⁺.
✗ Only OH⁻(aq) ions — the positive Na⁺ ions are set free at the very same moment; both ions always appear together.
📌 SSC & Railway Exam Facts
• NaOH = caustic soda, KOH = caustic potash, Ca(OH)₂ = slaked lime.
• All alkalis are bases, but all bases are not alkalis — an alkali is a water-soluble base.
• Acids give H⁺ (H₃O⁺) ions in water; bases give OH⁻ ions.
• NaOH is made by the electrolysis of brine (chlor-alkali process): 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
• In that process Cl₂ is liberated at the anode and H₂ at the cathode.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q106. The action of chlorine on dry Ca(OH)₂ produces which of the following?

  1. (a) Calcium chloride
  2. (b) Bleaching powder
  3. (c) Hydrochloric acid
  4. (d) Calcium oxide
Answer & solution

Answer: (b) Bleaching powder

Correct Answer: Option 2 — Bleaching powder
When chlorine gas (Cl₂) is passed over dry slaked lime, Ca(OH)₂, the product is bleaching powder, whose chemical name is calcium oxychloride and whose formula is written as CaOCl₂. The word dry in the question is the key — with a solution of slaked lime the reaction would go a different way.
Bleaching powder is a pale yellow powder with a strong smell of chlorine. It is a strong oxidising agent and releases available chlorine, which is what actually does the bleaching and the disinfecting.
Its uses: bleaching cotton, linen and wood pulp in the textile and paper industries, disinfecting drinking water, and making chloroform.
⚗️ The Reaction
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
• Slaked lime (dry) + chlorine → bleaching powder + water.
• Both sides carry 1 Ca, 2 Cl, 2 O and 2 H, so the equation is balanced.
• Bleaching powder bleaches by oxidation, so the whiteness it gives is permanent, unlike the temporary bleaching by SO₂.
❌ Why the other options are incorrect
✗ Calcium chloride (CaCl₂) — obtained when Ca(OH)₂ reacts with hydrochloric acid, not with chlorine gas: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.
✗ Hydrochloric acid (HCl) — formed by dissolving hydrogen chloride gas in water; the reaction of Cl₂ with dry slaked lime gives no acid.
✗ Calcium oxide (CaO) — quick lime, produced by heating limestone: CaCO₃ → (heat) CaO + CO₂; Ca(OH)₂ itself is made by adding water to CaO.
📌 SSC & Railway Exam Facts
• Bleaching powder = CaOCl₂ (calcium oxychloride), made from Cl₂ + dry Ca(OH)₂.
• CaO = quick lime, Ca(OH)₂ = slaked lime, CaCO₃ = limestone/marble/chalk.
• CaO + H₂O → Ca(OH)₂ + heat is a classic exothermic combination reaction.
• Bleaching powder is used to disinfect drinking water and to bleach wood pulp in paper mills.
• Plaster of Paris = CaSO₄·½H₂O, obtained by heating gypsum (CaSO₄·2H₂O) to 373 K.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · pH scale and its importance

Q107. Which of the following pairs is correct in terms of pH values?
(a) Tooth decay in humans: pH of mouth > 5.5
(b) pH range of human body: 7.0 to 7.8
(c) Acid rain: pH of rain water > 8

  1. (a) Only (a) is correct.
  2. (b) Only (b) is correct.
  3. (c) Only (c) is correct.
  4. (d) (a), (b) and (c) are correct.
Answer & solution

Answer: (b) Only (b) is correct.

Correct Answer: Option 2 — Only (b) is correct.
The pH scale runs from 0 to 14: pH < 7 is acidic, pH = 7 is neutral and pH > 7 is basic. It was given by Sørensen, and p stands for 'potenz' (power) — pH measures the concentration of H⁺ ions.
Statement (b) is correct. Our body works within a very narrow pH band of about 7.0 to 7.8; human blood pH is about 7.4. Rainwater or any fluid outside this band upsets the body's chemistry, which is why the range must be so tightly controlled.
Statement (a) is wrong. Tooth enamel — the hardest substance in the body, made of calcium phosphate — starts to corrode when the pH of the mouth falls below 5.5, not when it is above 5.5. Bacteria convert leftover sugar into acid, and toothpastes are basic so that they neutralise this excess acid.
Statement (c) is wrong. Rain is called acid rain when its pH is below 5.6, caused by oxides of sulphur and nitrogen (SO₂, NO₂) dissolving to give H₂SO₄ and HNO₃. A pH above 8 would make the rain basic.
❌ Why the other options are incorrect
✗ Only (a) is correct — decay begins below pH 5.5; the sign in (a) is reversed.
✗ Only (c) is correct — acid rain has pH < 5.6, never greater than 8.
✗ (a), (b) and (c) are correct — only (b) survives; both (a) and (c) state the inequality the wrong way round.
📌 SSC & Railway Exam Facts
• Tooth decay starts when mouth pH < 5.5; toothpaste is basic (alkaline).
• Human blood pH ≈ 7.4; saliva ≈ 6.5–7.5; gastric juice ≈ 1.2–3.0.
• Acid rain: pH < 5.6; normal rain is slightly acidic (pH ≈ 5.6) because of dissolved CO₂.
• Bee sting → methanoic (formic) acid, relieved by baking soda; nettle sting relieved by dock plant leaf.
• Plants grow best in soil of pH about 6.5–7.5; acidic soil is treated with quick lime or slaked lime.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q108. Which of the following statements is INCORRECT about the use of washing soda?

  1. (a) It is not used in the paper industry.
  2. (b) It is used in the manufacture of sodium compounds such as borax.
  3. (c) It is used for removing permanent hardness of water.
  4. (d) Sodium carbonate can be used as a cleaning agent for domestic purposes.
Answer & solution

Answer: (a) It is not used in the paper industry.

Correct Answer: Option 1 — It is not used in the paper industry.
Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O — a white crystalline solid that is basic in nature and is obtained from baking soda (NaHCO₃) by heating and then recrystallising.
The statement that it is not used in the paper industry is false. Washing soda is in fact used extensively in the glass, soap and paper industries, which is one of its standard NCERT-listed uses. Since the question asks for the incorrect statement, this is the answer.
The decahydrate form contains 10 molecules of water of crystallisation, which is why the solid looks dry yet the formula carries ·10H₂O. On heating it loses this water and becomes anhydrous sodium carbonate (soda ash).
⚗️ The Reaction
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
Washing soda removing permanent hardness:
Na₂CO₃ + CaCl₂ → CaCO₃↓ + 2NaCl
• The Ca²⁺ and Mg²⁺ ions that cause hardness are thrown out as insoluble carbonates.
❌ Why the other options are incorrect
✗ It is used in the manufacture of sodium compounds such as borax — this is a true statement; washing soda is a raw material for borax and other sodium salts, so it is not the answer.
✗ It is used for removing permanent hardness of water — true; it precipitates the calcium and magnesium salts responsible for hardness.
✗ Sodium carbonate can be used as a cleaning agent for domestic purposes — true; it is a well-known domestic cleaning agent, which is why it is called "washing" soda.
📌 SSC & Railway Exam Facts
• Washing soda = Na₂CO₃·10H₂O; baking soda = NaHCO₃; soda ash = anhydrous Na₂CO₃.
• Washing soda is used in the glass, soap and paper industries and to soften hard water.
• Baking soda + tartaric acid = baking powder; it releases CO₂ that makes the cake rise.
• NaHCO₃ is used in soda-acid fire extinguishers and as an antacid.
• Na₂CO₃ is made by the Solvay (ammonia-soda) process; on heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · Neutralisation reaction

Q109. Which of the following is obtained when dilute HCl is added to a beaker that contains copper oxide?

  1. (a) The blue-green colour of copper (II) chloride
  2. (b) The colourless solution of copper (II) chloride
  3. (c) The blue-green colour of copper (III) chloride
  4. (d) The red colour of copper (II) chloride
Answer & solution

Answer: (a) The blue-green colour of copper (II) chloride

Correct Answer: Option 1 — The blue-green colour of copper (II) chloride
Copper oxide (CuO) is a black metal oxide and, like all metal oxides, it is basic in nature. When dilute hydrochloric acid is added to it, a neutralisation takes place: the black powder dissolves and the solution slowly turns a blue-green (bluish-green) colour, which is the colour of copper(II) chloride, CuCl₂.
In CuO the copper has an oxidation state of +2, and this is carried over unchanged into the salt — hence copper(II) chloride, CuCl₂, and not copper(III) chloride. Copper practically never shows a +3 state in ordinary school chemistry.
This is the standard NCERT experiment used to prove that metal oxides are basic and that they react with acids just as bases do.
⚗️ The Reaction
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• Metal oxide + acid → salt + water — the same pattern as base + acid → salt + water.
• Two molecules of HCl are needed because copper is divalent (Cu²⁺), which balances the 2 Cl atoms.
• The black CuO disappears and a blue-green solution of CuCl₂ is left behind.
❌ Why the other options are incorrect
✗ The colourless solution of copper (II) chloride — CuCl₂ solution is not colourless; Cu²⁺ ions are coloured, giving a distinct blue-green tint.
✗ The blue-green colour of copper (III) chloride — the colour is right but the compound is wrong; copper is +2 here, so the salt is CuCl₂, i.e. copper(II) chloride.
✗ The red colour of copper (II) chloride — red is the colour of copper metal, not of its chloride solution; this is the common confusion between the metal and its salt.
📌 SSC & Railway Exam Facts
• Metal oxides are basic; non-metal oxides are acidic (CO₂, SO₂).
• CuO is black; CuCl₂ and CuSO₄ solutions are blue/blue-green; CuSO₄·5H₂O = blue vitriol.
• Al₂O₃ and ZnO are amphoteric — they react with both acids and bases.
• Acid + metal → salt + hydrogen; acid + metal carbonate → salt + water + CO₂.
• FeSO₄·7H₂O = green vitriol, ZnSO₄·7H₂O = white vitriol, FeSO₄ crystals are light green.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · pH scale and its importance

Q110. A student in a lab noted the pH value of a solution to be 10. Which of the following statements is correct about a solution?

  1. (a) A solution is basic with a high hydrogen ion concentration.
  2. (b) A solution is acidic with a high hydrogen ion concentration.
  3. (c) A solution is basic with a high hydroxide ion concentration.
  4. (d) A solution is acidic with a high hydroxide ion concentration.
Answer & solution

Answer: (c) A solution is basic with a high hydroxide ion concentration.

Correct Answer: Option 3 — A solution is basic with a high hydroxide ion concentration.
The pH scale runs from 0 to 14 and measures the concentration of hydrogen ions, H⁺ (H₃O⁺), in a solution:
pH < 7 → acidic (more H⁺ than OH⁻), pH = 7 → neutral (H⁺ = OH⁻), pH > 7 → basic (more OH⁻ than H⁺).
A solution of pH = 10 lies above 7, so it is basic (alkaline). As the pH rises above 7, the H⁺ concentration falls and the OH⁻ concentration rises — the two always move in opposite directions. Hence a pH-10 solution has a high hydroxide ion (OH⁻) concentration and a low hydrogen ion concentration.
Such a solution turns red litmus blue, gives a pink colour with phenolphthalein, tastes bitter and feels soapy. Common examples near pH 10 are milk of magnesia and a solution of washing soda.
❌ Why the other options are incorrect
✗ Basic with a high hydrogen ion concentration — the conclusion "basic" is right but the reason is wrong; a high H⁺ concentration means a low pH, i.e. an acid.
✗ Acidic with a high hydrogen ion concentration — this would be true only for pH < 7; a pH of 10 is on the basic side of neutral.
✗ Acidic with a high hydroxide ion concentration — self-contradictory; a high OH⁻ concentration makes a solution basic, never acidic.
📌 SSC & Railway Exam Facts
• pH = 0–7 acidic, 7 neutral, 7–14 basic; pH + pOH = 14.
• Lower pH → higher H⁺; higher pH → higher OH⁻; the two are inversely related.
• Indicators: litmus (red/blue), methyl orange (red in acid, yellow in base), phenolphthalein (colourless in acid, pink in base).
• Gastric juice pH ≈ 1.2, lemon ≈ 2.2, pure water = 7, milk of magnesia ≈ 10, NaOH ≈ 14.
• pH paper / universal indicator gives the approximate pH; onion and vanilla are olfactory indicators.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Properties of bases and alkalis

Q111. Which of the following statements about the reaction of a non-metallic oxide with a base is/are correct?
Statement I: The reaction of non-metallic oxide with base produces salt and water.
Statement II: Non-metallic oxides are acidic in nature.

  1. (a) Only statement II is correct.
  2. (b) Both statements I and II are correct.
  3. (c) Neither statement I nor II is correct.
  4. (d) Only statement I is correct.
Answer & solution

Answer: (b) Both statements I and II are correct.

Correct Answer: Option 2 — Both statements I and II are correct.
Statement II is correct. Non-metals form acidic oxides. When such an oxide dissolves in water it gives an acid — for example CO₂ + H₂O → H₂CO₃ (carbonic acid) and SO₂ + H₂O → H₂SO₃ (sulphurous acid). That is why non-metallic oxides turn blue litmus red.
Statement I is also correct. Being acidic, a non-metallic oxide neutralises a base, and every neutralisation gives salt and water. So the general rule is acidic oxide + base → salt + water, exactly parallel to acid + base → salt + water.
Since both statements are true, the answer is Both statements I and II are correct.
⚗️ The Reaction
CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)
• CO₂, a non-metallic oxide, behaves as an acid towards the base NaOH.
• The products are a salt (sodium carbonate) and water — proving Statement I.
• The classic laboratory test uses lime water: CO₂ + Ca(OH)₂ → CaCO₃(s) + H₂O, which turns it milky.
❌ Why the other options are incorrect
✗ Only statement II is correct — Statement I is equally true; an acidic oxide reacting with a base does yield salt + water.
✗ Neither statement I nor II is correct — both are standard NCERT statements, so this is plainly wrong.
✗ Only statement I is correct — Statement II is also true; the acidic nature of non-metallic oxides is the very reason Statement I works.
📌 SSC & Railway Exam Facts
• Metal oxides are basic (Na₂O, CaO, MgO); non-metal oxides are acidic (CO₂, SO₂, SO₃, NO₂, P₂O₅).
• Amphoteric oxides react with both acids and bases: Al₂O₃, ZnO, PbO, SnO.
• Neutral oxides are CO, N₂O, NO and H₂O.
• SO₂ and NO₂ dissolve in rain water to give H₂SO₄ and HNO₃ — the cause of acid rain.
• Acid + base → salt + water is a neutralisation and also a double displacement reaction.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Properties of acids

Q112. Which of the following is produced when acid reacts with metals?

  1. (a) Salt and hydrogen gas
  2. (b) Salt and water
  3. (c) Only Hydrogen gas
  4. (d) Only Salt
Answer & solution

Answer: (a) Salt and hydrogen gas

Correct Answer: Option 1 — Salt and hydrogen gas
The standard rule of the chapter is acid + metal → salt + hydrogen gas. A metal placed above hydrogen in the reactivity series displaces the H⁺ ions of a dilute acid, so hydrogen escapes as a gas and the metal takes its place to form a salt.
The evolved gas is identified by the classic pop test: bring a burning candle or splinter near the mouth of the tube and the hydrogen burns with a squeaky pop sound.
⚗️ The Reaction
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from hydrochloric acid.
• The salt formed is zinc chloride; brisk effervescence of H₂ is seen.
• Other examples: Mg + H₂SO₄ → MgSO₄ + H₂ and 2Na + 2HCl → 2NaCl + H₂.
❌ Why the other options are incorrect
✗ Salt and water — these are the products of neutralisation, i.e. acid + base → salt + water, not of an acid reacting with a metal.
✗ Only Hydrogen gas — the answer is incomplete; along with hydrogen a salt of the metal is always formed and stays in solution.
✗ Only Salt — also incomplete; the brisk effervescence of hydrogen is the very sign that the reaction is taking place.
📌 SSC & Railway Exam Facts
• Acid + metal → salt + H₂; acid + metal carbonate → salt + water + CO₂; acid + metal hydrogen carbonate → salt + water + CO₂; acid + base → salt + water.
• Cu, Hg, Ag and Au lie below hydrogen and do not liberate H₂ from dilute acids.
• Dilute nitric acid normally gives no H₂ — being an oxidising acid it gives oxides of nitrogen; magnesium and manganese are the exceptions.
• Hydrogen is confirmed by the pop sound test; CO₂ is confirmed by turning lime water milky.
• Metal oxides are basic, so metal oxide + acid → salt + water as well.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q113. Which of the following products is NOT formed when electricity is passed through an aqueous solution of sodium chloride?

  1. (a) Hydrogen gas
  2. (b) Chlorine gas
  3. (c) Oxygen gas
  4. (d) Sodium hydroxide
Answer & solution

Answer: (c) Oxygen gas

Correct Answer: Option 3 — Oxygen gas
Passing electricity through a concentrated aqueous solution of sodium chloride (brine) is the industrially important chlor-alkali process. It gives exactly three products, and oxygen is not one of them — so Option 3 is the required answer.
The name itself is a memory aid: "chlor" stands for the chlorine liberated and "alkali" for the sodium hydroxide produced.
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Chlorine gas (Cl₂) is released at the anode.
• Hydrogen gas (H₂) is released at the cathode.
• Sodium hydroxide (NaOH) is left near the cathode in the solution.
• Only H₂ and Cl₂ are evolved as gases; no O₂ is liberated, because the Cl⁻ ions of the concentrated brine are discharged at the anode in preference to water.
❌ Why the other options are incorrect
✗ Hydrogen gas — it is formed, at the cathode; it is used as a fuel, for making ammonia and for hydrogenating oils into vanaspati ghee.
✗ Chlorine gas — it is formed, at the anode; it is used for disinfecting drinking water and for making PVC, CFCs and pesticides.
✗ Sodium hydroxide — it is formed in the solution; caustic soda is used in soaps and detergents, paper making and degreasing metals.
📌 SSC & Railway Exam Facts
• Chlor-alkali process: brine → NaOH + Cl₂ + H₂; Cl₂ at anode, H₂ at cathode.
• Oxygen is obtained by the electrolysis of water (2H₂O → 2H₂ + O₂), where H₂ and O₂ are collected in the ratio 2 : 1.
• NaOH = caustic soda; NaCl = common salt (rock salt); Na₂CO₃·10H₂O = washing soda; NaHCO₃ = baking soda.
• Cl₂ + dry Ca(OH)₂ → CaOCl₂ + H₂O gives bleaching powder.
• Common salt is the raw material for NaOH, washing soda, baking soda and bleaching powder.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q114. What product is formed in the chlor-alkali process?

  1. (a) Sodium Hydroxide
  2. (b) Baking Soda
  3. (c) Washing Soda
  4. (d) Sodium Chloride
Answer & solution

Answer: (a) Sodium Hydroxide

Correct Answer: Option 1 — Sodium Hydroxide
In the chlor-alkali process, electricity is passed through a concentrated aqueous solution of sodium chloride (brine). The process is named chlor-alkali because its two chief products are chlorine and the alkali sodium hydroxide (NaOH).
Sodium chloride here is the raw material, not the product — it is decomposed. Chlorine gas is liberated at the anode, hydrogen gas at the cathode, and sodium hydroxide solution collects near the cathode.
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Cl₂ → anode, H₂ → cathode, NaOH → near the cathode.
• Check the balance — 2 Na, 2 Cl, 4 H and 2 O on both sides.
• Uses: NaOH for soaps, detergents and paper; Cl₂ for bleaching powder, PVC and water treatment; H₂ for fuel and margarine.
❌ Why the other options are incorrect
✗ Baking Soda — sodium hydrogencarbonate, NaHCO₃, is made by Solvay's process from brine, ammonia and CO₂, not by electrolysis.
✗ Washing Soda — Na₂CO₃·10H₂O is obtained by heating baking soda to Na₂CO₃ and then recrystallising it with water.
✗ Sodium Chloride — NaCl is the reactant (brine) that is electrolysed; it is consumed, not formed.
📌 SSC & Railway Exam Facts
• Brine = concentrated NaCl solution; its electrolysis is the chlor-alkali process.
• NaOH = caustic soda; NaHCO₃ = baking soda; Na₂CO₃·10H₂O = washing soda.
• Cl₂ + dry slaked lime Ca(OH)₂ → bleaching powder, CaOCl₂.
• H₂ + Cl₂ → 2HCl, used in the PVC and pharmaceutical industry.
• Common salt is the starting material for NaOH, NaHCO₃, Na₂CO₃ and bleaching powder alike.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q115. What is used in the preparation of calcium sulphate hemihydrate?

  1. (a) Gypsum
  2. (b) Sodium Carbonate
  3. (c) Copper Sulphate
  4. (d) Sodium Hydrogen Carbonate
Answer & solution

Answer: (a) Gypsum

Correct Answer: Option 1 — Gypsum
Calcium sulphate hemihydrate, CaSO₄·½H₂O, is the chemical name of Plaster of Paris. It is made by heating gypsum (CaSO₄·2H₂O) at 373 K (100 °C), at which temperature gypsum loses three-quarters of its water of crystallisation, leaving only half a molecule of water per formula unit — hence "hemi" (half) hydrate.
The temperature must be controlled carefully. Above 373 K the whole of the water is driven off, giving anhydrous calcium sulphate (CaSO₄), called dead burnt plaster, which will not set with water and is useless for plastering.
Plaster of Paris is a white powder that, on mixing with water, sets into a hard solid mass in a few minutes by taking the water back and turning into gypsum again. It is used for plaster casts for fractured bones, for making toys and statues, for decorative false ceilings and for smoothing wall surfaces.
⚗️ The Reactions
CaSO₄·2H₂O → (heat, 373 K) CaSO₄·½H₂O + 1½H₂O
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (sets hard)
• Setting is simply the re-formation of gypsum, and it is exothermic.
• Plaster of Paris must be stored in a moisture-proof container, or it sets in the packet.
❌ Why the other options are incorrect
✗ Sodium Carbonate — Na₂CO₃·10H₂O (washing soda) is used in laundry and to remove permanent hardness of water; it contains no calcium sulphate.
✗ Copper Sulphate — CuSO₄·5H₂O (blue vitriol) is a copper compound used in electroplating and as a fungicide; heating it gives white anhydrous CuSO₄, not Plaster of Paris.
✗ Sodium Hydrogen Carbonate — NaHCO₃ (baking soda) is used in baking powder and antacids; on heating it gives Na₂CO₃, H₂O and CO₂.
📌 SSC & Railway Exam Facts
• Gypsum = CaSO₄·2H₂O; Plaster of Paris = CaSO₄·½H₂O; heating temperature 373 K (100 °C).
• Heating above 373 K gives dead burnt plaster (anhydrous CaSO₄), which does not set.
• Plaster of Paris is used for plaster casts, statues, toys and false ceilings.
• Gypsum is added to cement (about 2–3%) to slow down its rate of setting.
• Other hydrated salts: CuSO₄·5H₂O (blue vitriol), FeSO₄·7H₂O (green vitriol), Na₂CO₃·10H₂O (washing soda).
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q116. Which of the following edible acids is added to baking soda for making baking powder?

  1. (a) Sodium chloride
  2. (b) Vinegar
  3. (c) Tartaric acid
  4. (d) Lactic acid
Answer & solution

Answer: (c) Tartaric acid

Correct Answer: Option 3 — Tartaric acid
Baking powder = baking soda (sodium hydrogencarbonate, NaHCO₃) + a mild edible acid, and the acid normally used is tartaric acid.
When baking powder is mixed into the dough and heated, the NaHCO₃ decomposes to give carbon dioxide. The CO₂ bubbles get trapped in the dough and make the cake rise and turn soft and spongy.
If baking soda alone were used, the decomposition would leave behind sodium carbonate (Na₂CO₃), which is strongly alkaline and makes the cake bitter in taste. The tartaric acid neutralises this sodium carbonate, so the bitterness never appears. That is the whole purpose of adding the acid.
⚗️ The Reaction
NaHCO₃ + H⁺ (from tartaric acid) → CO₂↑ + H₂O + sodium salt of the acid
• On heating alone: 2NaHCO₃ → (heat) Na₂CO₃ + H₂O + CO₂↑ — the Na₂CO₃ left behind is what tastes bitter.
• With tartaric acid present, that Na₂CO₃ is neutralised, leaving only sodium tartrate, which is harmless.
• The trapped CO₂ is what makes bread, cake and idli rise.
❌ Why the other options are incorrect
✗ Sodium chloride — common salt (NaCl) is a neutral salt, not an acid; it only adds taste and cannot neutralise sodium carbonate.
✗ Vinegar — vinegar is a dilute (5–8%) liquid solution of ethanoic acid; being a liquid it cannot be packed in a dry powder, though it does react with baking soda in the classic fizzing experiment.
✗ Lactic acid — the acid of curd and sour milk, also formed in muscles during heavy exercise; it is not the acid used in commercial baking powder.
📌 SSC & Railway Exam Facts
• Baking soda = NaHCO₃ (sodium hydrogencarbonate/bicarbonate), mildly alkaline, also used in soda-acid fire extinguishers and as an antacid.
• Washing soda = Na₂CO₃·10H₂O; bleaching powder = CaOCl₂; plaster of Paris = CaSO₄·½H₂O.
• Common acids: citric acid — lemon, lactic acid — curd, acetic acid — vinegar, oxalic acid — tomato, tartaric acid — tamarind, formic acid — ant sting.
• CO₂ turns lime water milky; this is its confirmatory test.
• All three — baking soda, washing soda and bleaching powder — are made from common salt (NaCl).
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Neutralisation reaction

Q117. What term is used for the reaction between acids and bases that results in the formation of salt and water?

  1. (a) Neutralisation reaction
  2. (b) Polymerisation reaction
  3. (c) Photosynthesis reaction
  4. (d) Hydrolysis reaction
Answer & solution

Answer: (a) Neutralisation reaction

Correct Answer: Option 1 — Neutralisation reaction
The reaction in which an acid reacts with a base to give a salt and water is called a neutralisation reaction. Its general form is:
Acid + Base → Salt + Water
At the ionic level, the H⁺ ions of the acid combine with the OH⁻ ions of the base to form water, so the acidic and basic characters cancel each other out — the mixture becomes neutral, with pH close to 7. Neutralisation is always an exothermic reaction (heat is released) and is also a form of double displacement.
Everyday uses are everywhere: antacids (Mg(OH)₂, NaHCO₃) neutralise excess stomach acid, baking soda on a bee sting relieves the formic acid, lime is added to acidic soil, and calamine (zinc carbonate) soothes an ant bite.
⚗️ The Reaction
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• The acid's H⁺ and the base's OH⁻ unite: H⁺ + OH⁻ → H₂O.
• The leftover Na⁺ and Cl⁻ form the salt, sodium chloride.
• Another standard example: Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O.
❌ Why the other options are incorrect
✗ Polymerisation reaction — the joining of many small monomers into a giant molecule (ethene → polythene); no acid, base, salt or water is involved.
✗ Photosynthesis reaction — a biological process in green plants: 6CO₂ + 6H₂O → (sunlight, chlorophyll) C₆H₁₂O₆ + 6O₂; it makes glucose, not a salt.
✗ Hydrolysis reaction — here a compound is broken down by water, e.g. a salt of a weak acid dissolving to give an alkaline solution; it is the reverse idea, not acid meeting base.
📌 SSC & Railway Exam Facts
• Acid + base → salt + water is neutralisation, an exothermic double displacement reaction.
• pH scale runs 0–14: < 7 acidic, = 7 neutral, > 7 basic; it was given by S. P. L. Sørensen.
• Acid + metal → salt + H₂; acid + metal carbonate → salt + water + CO₂.
• Strong acid + strong base → neutral salt (NaCl); strong acid + weak base → acidic salt (NH₄Cl); weak acid + strong base → basic salt (Na₂CO₃).
• Tooth decay begins below pH 5.5; toothpastes are basic to neutralise the acid formed in the mouth.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Properties of acids

Q118. Which of the following statements is correct when an acid is dissolved in water?
a) The separation of H⁺ ions from HCl molecules cannot occur in the absence of water.
b) Hydrogen ions must always be shown as H⁺ (aq) or hydronium ion (H₃O⁺).

  1. (a) Neither a nor b
  2. (b) Only a
  3. (c) Only b
  4. (d) Both a and b
Answer & solution

Answer: (d) Both a and b

Correct Answer: Option 4 — Both a and b
Statement (a) is correct. An acid shows its acidic behaviour only in the presence of water. Dry HCl gas does not ionise — it does not turn dry blue litmus red and does not conduct electricity. Water molecules are needed to pull the H⁺ ion away from the chloride, because the separation of ions requires the large energy of hydration supplied by water. That is exactly why dry HCl cannot behave as an acid.
Statement (b) is also correct. A bare H⁺ ion is simply a proton and is far too small and highly charged to exist alone in solution. It immediately attaches itself to a water molecule to form the hydronium ion, H₃O⁺. Hence hydrogen ions in an aqueous solution must always be written as H⁺(aq) or as H₃O⁺.
Both statements are the standard NCERT statements, so the answer is Both a and b.
⚗️ The Reaction
HCl + H₂O → H₃O⁺ + Cl⁻
• The H⁺ released by HCl combines with H₂O to give the hydronium ion, H₃O⁺.
• H⁺ + H₂O → H₃O⁺ is therefore the correct way to show a hydrogen ion in water.
• Dissolving an acid in water is highly exothermic — always add acid slowly to water, never water to acid.
❌ Why the other options are incorrect
✗ Neither a nor b — both statements are true, so this is completely wrong.
✗ Only a — statement (b) is equally true; a free proton cannot exist in water and becomes H₃O⁺.
✗ Only b — statement (a) is equally true; dry HCl gas does not ionise and so shows no acidic character.
📌 SSC & Railway Exam Facts
• Acids produce H⁺(aq)/H₃O⁺ ions in water; bases produce OH⁻ ions — the Arrhenius definition.
• Dry HCl gas does not turn dry blue litmus red; moist litmus is needed.
• The dissolving of an acid or base in water is called dilution and is a highly exothermic process.
• Acids and bases conduct electricity in aqueous solution because they furnish free ions.
• Strong acids (HCl, H₂SO₄, HNO₃) ionise completely; weak acids (CH₃COOH, H₂CO₃) ionise partially.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · Properties of acids

Q119. Which of the following is a characteristic property of acids?

  1. (a) They have a pH greater than 7.
  2. (b) They turn blue litmus paper red.
  3. (c) They turn red litmus paper blue.
  4. (d) They have a slippery feel.
Answer & solution

Answer: (b) They turn blue litmus paper red.

Correct Answer: Option 2 — They turn blue litmus paper red.
An acid is a substance that gives hydrogen ions, H⁺ (or H₃O⁺), in aqueous solution. It is these H⁺ ions that produce every typical acidic property — a sour taste, a pH below 7, the ability to conduct electricity, and the reddening of blue litmus.
Litmus is a natural indicator obtained from lichens. The rule to memorise is simple: acid turns blue litmus red, and base turns red litmus blue. A base also feels soapy or slippery and tastes bitter.
Acids also react with active metals to release hydrogen gas, and with carbonates to release carbon dioxide — both classic laboratory tests.
⚗️ The Reaction
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• An acid supplies H⁺ ions, which are reduced by the metal to give hydrogen gas.
• The gas burns with a pop sound — the standard test for H₂.
• With a carbonate: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂, and the CO₂ turns lime water milky.
❌ Why the other options are incorrect
✗ They have a pH greater than 7 — a pH above 7 means basic; acids always have pH < 7 (pH = 7 is neutral).
✗ They turn red litmus paper blue — this is the test for a base, not an acid.
✗ They have a slippery feel — the soapy, slippery touch belongs to bases such as NaOH and soap solution.
📌 SSC & Railway Exam Facts
• Acid → blue litmus red; base → red litmus blue.
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic; the pH scale was given by S. P. L. Sørensen.
• Phenolphthalein: colourless in acid, pink in base; methyl orange: red in acid, yellow in base.
• Acid in the stomach is hydrochloric acid (HCl); lemon — citric acid, vinegar — acetic acid, curd — lactic acid.
• Onion and vanilla are olfactory indicators — they change smell, not colour.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q120. Which of the following is the most common application of plaster of Paris?

  1. (a) Fire extinguisher
  2. (b) Remove the hardness of water
  3. (c) Creating casts for supporting fractured bones
  4. (d) Baking
Answer & solution

Answer: (c) Creating casts for supporting fractured bones

Correct Answer: Option 3 — Creating casts for supporting fractured bones
Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is made by heating gypsum (CaSO₄·2H₂O) to about 373 K (100 °C), which drives off three-quarters of the water of crystallisation. Heating above this temperature must be avoided, because the anhydrous CaSO₄ (dead burnt plaster) formed then will no longer set.
When plaster of Paris is mixed with water it takes the water back and changes into gypsum again, setting within minutes into a hard, solid mass that expands slightly and does not shrink. This is why doctors use it in orthopaedic plaster casts to hold a fractured bone in position. It is also used for making statues, decorative mouldings, toys and blackboard chalk, and for fireproofing materials.
⚗️ The Reaction
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
• Setting is simply the rehydration of the hemihydrate back to gypsum.
• Preparation: CaSO₄·2H₂O → (373 K) CaSO₄·½H₂O + 1½H₂O.
• It must be stored in a moisture-proof container, or it sets in the bag.
❌ Why the other options are incorrect
✗ Fire extinguisher — this uses sodium hydrogen carbonate (baking soda, NaHCO₃), which releases CO₂ to smother the flame.
✗ Remove the hardness of water — washing soda (Na₂CO₃·10H₂O) removes hardness by precipitating Ca²⁺ and Mg²⁺ ions.
✗ Baking — the baking agent is baking soda (NaHCO₃) or baking powder, which liberates CO₂ and makes the cake rise.
📌 SSC & Railway Exam Facts
• Plaster of Paris = CaSO₄·½H₂O; gypsum = CaSO₄·2H₂O.
• It is prepared by heating gypsum to 373 K (100 °C); overheating gives dead burnt plaster.
• Baking soda = NaHCO₃, washing soda = Na₂CO₃·10H₂O, bleaching powder = CaOCl₂.
• Plaster of Paris gets its name from the large gypsum deposits at Montmartre, Paris.
• Antacids contain Mg(OH)₂ (milk of magnesia) or NaHCO₃ to neutralise excess stomach acid.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · pH scale and its importance

Q121. The pH value of the neutral solution is _______.

  1. (a) 7
  2. (b) 5
  3. (c) 14
  4. (d) 0
Answer & solution

Answer: (a) 7

Correct Answer: Option 1 — 7
The pH scale measures the concentration of hydrogen ions (H⁺) in a solution. The letter p stands for the German *potenz* (power), so pH means the 'power of hydrogen'. The scale runs from 0 to 14 at 25 °C.
A solution is neutral when the concentration of H⁺ ions is exactly equal to the concentration of OH⁻ ions. In pure water at 25 °C both are 10⁻⁷ mol/L, which gives pH = 7. Hence a neutral solution such as pure water, common salt solution (NaCl) or glucose solution has a pH of 7.
pH below 7 → acidic (the smaller the number, the stronger the acid) and pH above 7 → basic (the larger the number, the stronger the base). Each unit change in pH means a ten-fold change in H⁺ concentration.
❌ Why the other options are incorrect
✗ 5 — a pH of 5 is acidic; black coffee (≈5) and acid rain (< 5.6) fall in this region.
✗ 14 — this is the upper end of the scale, the pH of a strongly basic solution such as concentrated sodium hydroxide.
✗ 0 — this is the lower end of the scale, the pH of a very strong acid such as concentrated hydrochloric acid.
📌 SSC & Railway Exam Facts
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic; scale range 0–14.
• The pH scale was introduced by the Danish chemist S. P. L. Sørensen (1909).
• Human blood pH = 7.35–7.45 (slightly basic); saliva ≈ 6.5–7.5; gastric juice ≈ 1.2–3.0.
• Tooth decay begins when the pH in the mouth falls below 5.5; enamel is calcium phosphate, the hardest substance in the body.
• Rain is called acid rain when its pH is below 5.6; plants grow best in soil of pH ≈ 6.5–7.5.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · Neutralisation reaction

Q122. What compound is formed when a small amount of hydrochloric acid is added to a beaker containing copper oxide?

  1. (a) Copper (IV) chloride
  2. (b) Copper (I) chloride
  3. (c) Copper (II) chloride
  4. (d) Copper (III) chloride
Answer & solution

Answer: (c) Copper (II) chloride

Correct Answer: Option 3 — Copper (II) chloride
Copper oxide (CuO) is a black, basic metal oxide. A basic oxide reacts with an acid to give a salt and water — a neutralisation that is also a double displacement reaction.
In CuO, oxygen has an oxidation number of −2, so copper must be +2. When the acid replaces the oxide ion, copper keeps this +2 state, and each Cu²⁺ ion combines with two Cl⁻ ions to give CuCl₂ — copper(II) chloride. The Roman numeral (II) in the name is simply the oxidation state of copper.
The experiment is a favourite in textbooks because of the colour change: the black copper oxide dissolves and the solution turns a blue-green colour, which is the colour of copper(II) chloride in water.
⚗️ The Reaction
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• Metal oxide + acid → salt + water — a neutralisation reaction.
• Cu²⁺ needs two Cl⁻ ions to balance its charge, giving the formula CuCl₂.
• The black solid disappears and a blue-green solution is formed.
❌ Why the other options are incorrect
✗ Copper (IV) chloride — would need Cu⁴⁺ and the formula CuCl₄; copper does not show a stable +4 state.
✗ Copper (I) chloride — CuCl is formed from copper(I) oxide, Cu₂O (red), not from the black CuO given here.
✗ Copper (III) chloride — CuCl₃ is not an ordinary compound; the common oxidation states of copper are only +1 and +2.
📌 SSC & Railway Exam Facts
• Acid + metal oxide → salt + water; metal oxides are basic, non-metal oxides are acidic.
• Roman numerals in a name give the oxidation state: FeCl₂ iron(II) chloride, FeCl₃ iron(III) chloride.
• CuO is black, Cu₂O is red; CuSO₄·5H₂O (blue vitriol) is blue and turns white on heating.
• Acid + metal → salt + H₂; acid + carbonate → salt + water + CO₂.
• Copper: symbol Cu (from *cuprum*), atomic number 29; a very good conductor used in wiring.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Neutralisation reaction

Q123. Which of the following is produced when carbon dioxide reacts with calcium hydroxide?

  1. (a) H₂CO₃ + H₂O
  2. (b) CO + H₂O
  3. (c) CaCO₃ + H₂O
  4. (d) CaO + H₂O
Answer & solution

Answer: (c) CaCO₃ + H₂O

Correct Answer: Option 3 — CaCO₃ + H₂O
When carbon dioxide is passed through calcium hydroxide solution (lime water), the two react to give calcium carbonate and water. Calcium carbonate is insoluble in water, so it appears as a fine white precipitate that makes the clear lime water turn milky. This is the standard laboratory test for carbon dioxide gas.
CO₂ is an acidic oxide and Ca(OH)₂ is a base, so this is in effect a neutralisation: an acidic oxide + base → salt + water.
If excess CO₂ is passed on, the milkiness disappears again, because the insoluble CaCO₃ dissolves as soluble calcium hydrogencarbonate, Ca(HCO₃)₂.
⚗️ The Reaction
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l)
• Balance check — Ca: 1 = 1, C: 1 = 1, O: 4 = 4, H: 2 = 2. The equation is balanced as written.
• The (s) and the white milkiness show that CaCO₃ comes out as a precipitate.
• With excess CO₂: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂, and the milkiness vanishes.
• This is also the reaction that hardens whitewash — the CaCO₃ film gives the shiny white finish on a wall.
❌ Why the other options are incorrect
✗ H₂CO₃ + H₂O — carbonic acid is formed when CO₂ dissolves in water alone (CO₂ + H₂O → H₂CO₃, as in soda water), not when it reacts with a base.
✗ CO + H₂O — carbon monoxide is a product of the incomplete combustion of carbon; CO₂ is not reduced to CO by a base.
✗ CaO + H₂O — CaO (quick lime) with water is the reverse change: it produces Ca(OH)₂ rather than being produced from it.
📌 SSC & Railway Exam Facts
• Lime water turning milky is the test for CO₂; the milkiness is insoluble CaCO₃.
• Excess CO₂ removes the milkiness by forming soluble Ca(HCO₃)₂.
• CaCO₃ is the formula of limestone, marble and chalk; heating it gives CaO + CO₂ (decomposition).
• Ca(HCO₃)₂ and Mg(HCO₃)₂ cause the temporary hardness of water, removed by boiling.
• Whitewashing uses Ca(OH)₂, which sets into a shiny CaCO₃ coat with the CO₂ of the air.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · pH scale and its importance

Q124. The pH of gastric juice is ______.

  1. (a) 10.0
  2. (b) 7.4
  3. (c) 2.2
  4. (d) 1.2
Answer & solution

Answer: (d) 1.2

Correct Answer: Option 4 — 1.2
Gastric juice is the digestive fluid secreted by the glands in the wall of the stomach. It contains hydrochloric acid (HCl), the enzyme pepsin and mucus. Because of the HCl, gastric juice is strongly acidic, and the value learnt from NCERT is pH ≈ 1.2.
This strong acidity serves two purposes: it kills the bacteria swallowed with food, and it provides the acidic medium that pepsin needs to digest proteins. The stomach wall is protected from this acid by a thick layer of mucus.
When too much acid is secreted, it causes acidity and indigestion, treated with antacids — mild bases such as milk of magnesia, Mg(OH)₂, or sodium bicarbonate, NaHCO₃ — which neutralise the excess HCl.
❌ Why the other options are incorrect
✗ 10.0 — a distinctly basic value; nothing in the stomach is alkaline. A pH near 10 belongs to solutions such as milk of magnesia.
✗ 7.4 — this is the pH of human blood (7.35–7.45), which is slightly basic and very tightly controlled. It is the standard distractor here.
✗ 2.2 — acidic, but not the textbook figure; NCERT gives the pH of gastric juice as 1.2, so this value is not the one asked for.
📌 SSC & Railway Exam Facts
• pH = −log[H⁺]; the scale runs from 0 to 14 and was given by S. P. L. Sørensen.
• Standard pH values: gastric juice 1.2, lemon juice 2.2, vinegar 3.0, milk 6.6, pure water 7, blood 7.4, baking soda solution 9, milk of magnesia 10, NaOH 14.
• Tooth decay starts below pH 5.5; toothpastes are basic to neutralise mouth acids.
• Bee sting is acidic — treated with baking soda; wasp sting is basic — treated with vinegar.
• Acid rain has pH below 5.6, caused by SO₂ and NO₂ dissolving in rainwater.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Properties of acids

Q125. In an aqueous solution, acid and base produce:

  1. (a) K⁺ ions and CO₃²⁻ ions, respectively
  2. (b) H⁺ ions and OH⁻ ions, respectively
  3. (c) Li⁺ ions and CO₃²⁻ ions, respectively
  4. (d) Na⁺ ions and CO₃²⁻ ions, respectively
Answer & solution

Answer: (b) H⁺ ions and OH⁻ ions, respectively

Correct Answer: Option 2 — H⁺ ions and OH⁻ ions, respectively
According to the Arrhenius theory, an acid is a substance that gives hydrogen ions, H⁺, in aqueous solution, and a base is one that gives hydroxide ions, OH⁻, in aqueous solution.
⚗️ The Ionisation
HCl(aq) → H⁺(aq) + Cl⁻(aq) — an acid
NaOH(aq) → Na⁺(aq) + OH⁻(aq) — a base
• The free H⁺ ion never exists alone in water; it attaches to a water molecule to form the hydronium ion, H₃O⁺.
• Acids and bases conduct electricity in aqueous solution precisely because of these free ions.
• Dry HCl gas does not turn dry blue litmus red — it must have water to release H⁺, which proves that ionisation needs water.
When the two are mixed, H⁺ + OH⁻ → H₂O, which is the essence of a neutralisation reaction.
❌ Why the other options are incorrect
✗ K⁺ ions and CO₃²⁻ ions — K⁺ comes from a potassium salt or from KOH, and CO₃²⁻ from a carbonate; neither ion defines an acid or a base.
✗ Li⁺ ions and CO₃²⁻ ions — lithium and carbonate ions are found in lithium carbonate, an ordinary salt, and have nothing to do with the definition.
✗ Na⁺ ions and CO₃²⁻ ions — these two make up sodium carbonate (washing soda), which is a salt, even though its solution happens to be basic.
📌 SSC & Railway Exam Facts
• Arrhenius — acid gives H⁺, base gives OH⁻ in water.
• Bronsted–Lowry — acid is a proton donor, base a proton acceptor; Lewis — acid is an electron-pair acceptor, base a donor.
• pH = −log[H⁺]; pH < 7 acid, = 7 neutral, > 7 base.
• Strong acids: HCl, H₂SO₄, HNO₃; weak acids: CH₃COOH, H₂CO₃. Strong bases: NaOH, KOH; weak base: NH₄OH.
• Always add acid to water, never water to acid — the dilution is highly exothermic and can cause the mixture to splash.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q126. The Chlor-alkali process is used for the formation of which of the following?

  1. (a) Sodium hydroxide
  2. (b) Sodium carbonate
  3. (c) Sodium chloride
  4. (d) Sodium bicarbonate
Answer & solution

Answer: (a) Sodium hydroxide

Correct Answer: Option 1 — Sodium hydroxide
The chlor-alkali process is the industrial electrolysis of a concentrated aqueous solution of sodium chloride, called brine. The name itself is the clue: “chlor” for the chlorine produced and “alkali” for the alkali sodium hydroxide (NaOH) produced.
When electric current is passed through brine, chlorine gas (Cl₂) is liberated at the anode, hydrogen gas (H₂) at the cathode, and sodium hydroxide solution is left behind near the cathode. All three products are commercially valuable, which makes the process highly economical.
Their uses: NaOH for soaps, detergents, paper and degreasing metals; Cl₂ for water treatment, PVC, CFCs and bleaching powder; H₂ for fuels, margarine and ammonia (fertilisers).
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Cl₂ at the anode, H₂ at the cathode, NaOH remains in the solution near the cathode.
• Check the balance: 2 Na, 2 Cl, 4 H and 2 O appear on both sides.
• Cl₂ + dry Ca(OH)₂ → CaOCl₂ + H₂O turns the chlorine into bleaching powder.
❌ Why the other options are incorrect
✗ Sodium carbonate — washing soda, Na₂CO₃·10H₂O, is made by the Solvay (ammonia–soda) process, not by electrolysis of brine.
✗ Sodium chloride — common salt is the raw material (brine) that is fed into the process, not the product.
✗ Sodium bicarbonate — baking soda, NaHCO₃, is made by passing CO₂ and NH₃ through brine: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.
📌 SSC & Railway Exam Facts
• Chlor-alkali process: electrolysis of brine gives NaOH + Cl₂ + H₂.
• NaOH = caustic soda; Na₂CO₃·10H₂O = washing soda; NaHCO₃ = baking soda; NaCl = common salt.
• CaOCl₂ = bleaching powder, made from Cl₂ + dry slaked lime.
• Anode → Cl₂ (oxidation); cathode → H₂ (reduction) — a standard PYQ pairing.
• Baking soda is a mild, non-corrosive base used in antacids and baking powder.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q127. Which products are formed in the chlor-alkali process when electricity is passed through an aqueous sodium chloride solution?
NaCl(aq) + H₂O(l) → Products

  1. (a) H₂ + O₂
  2. (b) NaHCO₃ + Cl₂
  3. (c) NaOH + H₂ + Cl₂
  4. (d) NaCl + H₂
Answer & solution

Answer: (c) NaOH + H₂ + Cl₂

Correct Answer: Option 3 — NaOH + H₂ + Cl₂
The chlor-alkali process is the industrial electrolysis of brine, a concentrated aqueous solution of sodium chloride. The name itself gives the products away — “chlor” for chlorine and “alkali” for the alkali sodium hydroxide.
When current is passed, the chloride ions (Cl⁻) are oxidised at the anode to give chlorine gas (Cl₂), water is reduced at the cathode to give hydrogen gas (H₂), and the Na⁺ and OH⁻ left in solution form sodium hydroxide (NaOH) near the cathode. All three products are commercially useful, which is what makes the process so economical.
Uses: NaOH in soaps, detergents, paper and de-greasing metals; Cl₂ in water treatment, PVC, CFCs and bleaching powder; H₂ as a fuel and for making ammonia and margarine.
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Check the balance: 2 Na, 2 Cl, 4 H and 2 O on each side.
• Cl₂ is released at the anode, H₂ at the cathode, and NaOH stays in the solution near the cathode.
• Note that no oxygen is produced; the oxygen of the water leaves as part of the OH⁻ in NaOH.
❌ Why the other options are incorrect
✗ H₂ + O₂ — these are the products of the electrolysis of pure (acidified) water, not of brine; in brine the Cl⁻ ion is discharged in preference to OH⁻.
✗ NaHCO₃ + Cl₂ — sodium bicarbonate is a product of the Solvay process (NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃), which needs CO₂ and NH₃, not electricity.
✗ NaCl + H₂ — NaCl is the reactant, so it cannot be listed as a product; also the essential products NaOH and Cl₂ are missing.
📌 SSC & Railway Exam Facts
• Chlor-alkali process: electrolysis of brine gives NaOH (cathode side) + Cl₂ (anode) + H₂ (cathode).
• NaOH = caustic soda; NaHCO₃ = baking soda; Na₂CO₃·10H₂O = washing soda; CaOCl₂ = bleaching powder.
• Cl₂ + dry Ca(OH)₂ → CaOCl₂ + H₂O — bleaching powder from the chlorine of this process.
• Oxidation always occurs at the anode, reduction at the cathode.
• Brine = saturated NaCl solution; common salt is obtained from sea water and rock salt deposits.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q128. Which of the following is treated with dry slaked lime [Ca(OH)₂] to make bleaching powder?

  1. (a) Carbon dioxide gas
  2. (b) Water
  3. (c) Hydrogen gas
  4. (d) Chlorine gas
Answer & solution

Answer: (d) Chlorine gas

Correct Answer: Option 4 — Chlorine gas
Bleaching powder (calcium oxychloride, CaOCl₂) is manufactured by passing chlorine gas over dry slaked lime, Ca(OH)₂. The chlorine supplied for this comes from the chlor-alkali process, the electrolysis of brine.
Bleaching powder is a pale yellowish white powder with a strong smell of chlorine. Its bleaching and germ-killing action is due to the available chlorine it releases; when it meets a dilute acid or even the CO₂ of moist air it slowly gives off Cl₂, which oxidises and thus destroys colouring matter and microorganisms.
Its uses are standard exam material: bleaching cotton, linen and wood pulp in textile and paper industries, disinfecting drinking water, and acting as an oxidising agent in chemical industries.
⚗️ The Reaction
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
• Dry slaked lime, Ca(OH)₂, absorbs chlorine to give bleaching powder, CaOCl₂.
• The lime must be dry — with wet lime the chlorine would form a mixture of calcium chloride and calcium hypochlorite instead.
• Count the atoms: 1 Ca, 2 O, 2 H, 2 Cl on both sides, so the equation is balanced as written.
❌ Why the other options are incorrect
✗ Carbon dioxide gas — CO₂ with lime water, Ca(OH)₂, gives a white precipitate of calcium carbonate (CaCO₃): Ca(OH)₂ + CO₂ → CaCO₃ + H₂O — that is the test for CO₂, not bleaching powder.
✗ Water — adding water to quick lime (CaO) is what produces slaked lime with a hissing sound (CaO + H₂O → Ca(OH)₂); water added to slaked lime merely gives lime water.
✗ Hydrogen gas — H₂ is a reducing agent and does not react with slaked lime to give any bleaching agent.
📌 SSC & Railway Exam Facts
• Bleaching powder = CaOCl₂ (calcium oxychloride), made from Cl₂ + dry Ca(OH)₂.
• CaO = quick lime, Ca(OH)₂ = slaked lime, CaCO₃ = limestone/marble/chalk, CaSO₄·½H₂O = plaster of Paris.
• CaO + H₂O → Ca(OH)₂ is a strongly exothermic (heat-releasing) reaction.
• Lime water turns milky with CO₂ due to CaCO₃, and clear again with excess CO₂ (soluble Ca(HCO₃)₂).
• Bleaching powder acts by releasing available chlorine; it is used to disinfect drinking water.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q129. Which of the following compounds is used in a soda-acid fire extinguisher?

  1. (a) Ca(OH)₂
  2. (b) H₂CO₃
  3. (c) Na₂CO₃
  4. (d) NaHCO₃
Answer & solution

Answer: (d) NaHCO₃

Correct Answer: Option 4 — NaHCO₃
NaHCO₃ is sodium hydrogencarbonate, better known as baking soda or sodium bicarbonate. It is a mild, non-corrosive base made by the Solvay process from brine, ammonia and carbon dioxide.
In a soda-acid fire extinguisher the cylinder holds a solution of sodium hydrogencarbonate and a sealed glass bottle of sulphuric acid. When the knob is struck, the bottle breaks, the acid mixes with the NaHCO₃ solution and a large volume of carbon dioxide (CO₂) is generated at once. The pressure of the gas drives the liquid out of the nozzle, and the CO₂ — being about 1.5 times heavier than air — blankets the burning material and cuts off its oxygen supply, putting the fire out.
The same gas-releasing reaction explains the other uses of baking soda: it makes cakes and bread rise, and as an antacid it neutralises excess stomach acid.
⚗️ The Reaction
2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂
• Sodium hydrogencarbonate reacts with the sulphuric acid released from the broken bottle to give CO₂ under pressure.
• Balance check: 2 Na, 2 H (from NaHCO₃) + 2 H (from acid), 1 S, 2 C and 10 O on each side.
• CO₂ neither burns nor supports burning and is heavier than air, so it smothers the flame.
❌ Why the other options are incorrect
✗ Ca(OH)₂ — slaked lime; used in whitewashing, in making bleaching powder and to neutralise acidic soil, not in extinguishers.
✗ H₂CO₃ — carbonic acid, the weak acid of soda water; it is unstable and decomposes, so it is not stored as the active compound of an extinguisher.
✗ Na₂CO₃ — washing soda (Na₂CO₃·10H₂O); used for softening hard water, in glass and soap manufacture and as a cleaning agent, not in soda-acid extinguishers.
📌 SSC & Railway Exam Facts
• NaHCO₃ = baking soda; Na₂CO₃·10H₂O = washing soda; NaOH = caustic soda; NaCl = common salt.
• Baking powder = baking soda + a mild edible acid (tartaric acid); the acid stops the cake tasting bitter.
• NaHCO₃ is used as an antacid because it is a mild base that neutralises excess HCl in the stomach.
• CO₂ extinguishers are the best for electrical and oil fires; water must never be used on them.
• On heating, 2NaHCO₃ → (heat) Na₂CO₃ + H₂O + CO₂ — a decomposition reaction.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 1 · pH scale and its importance

Q130. Which of the following has the highest basicity?

  1. (a) Tap water
  2. (b) Saliva
  3. (c) Blood
  4. (d) Sodium hydroxide
Answer & solution

Answer: (d) Sodium hydroxide

Correct Answer: Option 4 — Sodium hydroxide
Basicity is measured on the pH scale, which runs from 0 to 14. A pH below 7 is acidic, pH 7 is neutral and a pH above 7 is basic — the higher the pH, the stronger the base.
Sodium hydroxide (NaOH), also called caustic soda, is a strong base. It ionises completely in water to give a very high concentration of OH⁻ ions, and a concentrated solution has a pH of about 13–14 — the highest on the whole scale. It is so strongly alkaline that it is corrosive to skin.
Of the remaining choices, blood has the highest pH (7.35–7.45), but that is only very mildly basic compared with NaOH.
❌ Why the other options are incorrect
✗ Tap water — essentially neutral, with a pH close to 7; it is neither a base nor an acid.
✗ Saliva — its pH is about 6.5 to 7.5, so it is nearly neutral and even slightly acidic before a meal.
✗ Blood — buffered at pH 7.35–7.45, only slightly basic; the body keeps this narrow range strictly constant.
📌 SSC & Railway Exam Facts
• pH scale 0–14 was given by S. P. L. Sørensen; p stands for potenz (power) of hydrogen.
• Strong bases: NaOH, KOH, Ca(OH)₂; weak bases: NH₄OH, Mg(OH)₂.
• Gastric juice pH ≈ 1.2–3, lemon juice ≈ 2.2, milk ≈ 6.4, pure water = 7, blood ≈ 7.4.
• NaOH = caustic soda, KOH = caustic potash, Ca(OH)₂ = slaked lime.
• Tooth decay begins when the mouth pH falls below 5.5; toothpastes are mildly basic.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 1 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q131. In chemical industries, bleaching powder is used as:

  1. (a) a dehydrating agent
  2. (b) a reducing agent
  3. (c) an oxidising agent
  4. (d) a fire extinguisher
Answer & solution

Answer: (c) an oxidising agent

Correct Answer: Option 3 — an oxidising agent
Bleaching powder (calcium oxychloride, CaOCl₂) is made by passing chlorine over dry slaked lime:
⚗️ The Reaction
Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)
• In moist air it slowly releases chlorine, which with water gives nascent oxygen [O].
• It is this nascent oxygen that oxidises dyes, germs and stains — so bleaching powder acts as a powerful oxidising agent in many chemical industries.
Uses: bleaching cotton, linen and wood pulp in the textile and paper industries, disinfecting drinking water, and as an oxidising agent in chemical manufacture.
❌ Why the other options are incorrect
✗ a dehydrating agent — a dehydrating agent removes water from a substance; that is the job of concentrated H₂SO₄ or P₂O₅, not of bleaching powder.
✗ a reducing agent — a reducing agent donates electrons / removes oxygen (H₂, C, CO); bleaching powder does exactly the opposite, it supplies oxygen.
✗ a fire extinguisher — fire extinguishers use CO₂ or sodium bicarbonate; a chlorine-releasing oxidiser would feed a fire, not put it out.
📌 SSC & Railway Exam Facts
• Bleaching powder = CaOCl₂, made from Ca(OH)₂ + Cl₂; it smells strongly of chlorine.
• NaOH = caustic soda, Na₂CO₃·10H₂O = washing soda, NaHCO₃ = baking soda, CaSO₄·½H₂O = plaster of Paris.
• Chlorine for bleaching powder comes from the chlor-alkali process.
• SO₂ bleaches by reduction (temporary); bleaching powder and Cl₂ bleach by oxidation (permanent).
• Bleaching powder is used to make drinking water germ-free (chlorination).
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 1 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q132. Plaster of Paris is prepared when:

  1. (a) brine is heated at 373 K and it loses water molecules
  2. (b) ammonia is heated at 373 K and it loses water molecules
  3. (c) gypsum is heated at 373 K and it loses water molecules
  4. (d) soda lime is heated at 373 K and it loses water molecules
Answer & solution

Answer: (c) gypsum is heated at 373 K and it loses water molecules

Correct Answer: Option 3 — gypsum is heated at 373 K and it loses water molecules
Plaster of Paris (POP) is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is made by heating gypsum (calcium sulphate dihydrate, CaSO₄·2H₂O) to 373 K (100 °C), at which temperature gypsum loses three-fourths of its water of crystallisation.
⚗️ The Reaction
CaSO₄·2H₂O → (373 K) CaSO₄·½H₂O + 1½H₂O
• Only ½ molecule of water stays behind per formula unit, hence the name hemihydrate.
• On adding water back, POP sets into a hard solid mass of gypsum again — that is why it is used for plaster casts for fractured bones, and for making toys, statues, false ceilings and smooth wall surfaces.
• It must be stored in a moisture-proof container, or it sets on its own.
Heating above 373 K drives out all the water and gives dead burnt plaster, which will no longer set.
❌ Why the other options are incorrect
✗ brine — a concentrated solution of NaCl in water; heating it gives common salt, and it is the raw material of the chlor-alkali process, not of POP.
✗ ammonia — NH₃ is a gas containing no water of crystallisation at all, so it cannot lose water molecules.
✗ soda lime — a mixture of NaOH and CaO, used as a drying agent and in decarboxylation reactions; it does not yield plaster of Paris.
📌 SSC & Railway Exam Facts
• POP = CaSO₄·½H₂O; gypsum = CaSO₄·2H₂O; 373 K = 100 °C.
• Other common salts: NaHCO₃ baking soda, Na₂CO₃·10H₂O washing soda, CaOCl₂ bleaching powder, CuSO₄·5H₂O blue vitriol.
• POP is used in surgical bandages, moulds, statues and as a fire-proofing material.
• Gypsum is also added to cement to slow down its setting.
• Plaster of Paris gets its name from the large gypsum deposits at Montmartre, Paris.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q133. Identify the best method to prepare washing soda from sodium bicarbonate.

  1. (a) By heating sodium bicarbonate
  2. (b) By mixing sodium bicarbonate with water
  3. (c) By freezing sodium bicarbonate
  4. (d) By adding sodium bicarbonate to vinegar
Answer & solution

Answer: (a) By heating sodium bicarbonate

Correct Answer: Option 1 — By heating sodium bicarbonate
Sodium bicarbonate (NaHCO₃) is baking soda, and washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. On heating, baking soda undergoes thermal decomposition to give sodium carbonate, which is then recrystallised from water to obtain washing soda.
⚗️ The Reaction
2NaHCO₃ → (heat) Na₂CO₃ + H₂O + CO₂
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
• The first step is a thermal decomposition reaction and is endothermic.
• The CO₂ released in the first step is what makes cake and bread rise.
• The 10H₂O in the final product is the water of crystallisation, which gives washing soda its crystalline form.
Washing soda is a basic salt of a strong base (NaOH) and a weak acid (H₂CO₃), so its solution is alkaline with pH above 7. It is used to remove the permanent hardness of water, in the glass, soap and paper industries, and as a domestic cleaning agent.
❌ Why the other options are incorrect
✗ By mixing sodium bicarbonate with water — this merely dissolves the salt; it is a physical change and no sodium carbonate is formed.
✗ By freezing sodium bicarbonate — cooling supplies no energy for decomposition; decomposition needs heat, not the removal of it.
✗ By adding sodium bicarbonate to vinegar — this gives sodium acetate, water and carbon dioxide (NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂), not washing soda.
📌 SSC & Railway Exam Facts
• Washing soda — Na₂CO₃·10H₂O; Baking soda — NaHCO₃; Common salt — NaCl.
• Bleaching powder — CaOCl₂, made by Ca(OH)₂ + Cl₂; Plaster of Paris — CaSO₄·½H₂O.
• Both washing soda and baking soda are made from brine by the Solvay process.
• Baking soda plus a mild edible acid (tartaric acid) makes baking powder.
• Baking soda is used in soda-acid fire extinguishers and as an antacid to neutralise stomach acidity.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Neutralisation reaction

Q134. When hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), which of the following products is formed?

  1. (a) Only water
  2. (b) Water and salt
  3. (c) Only salt
  4. (d) Carbon dioxide and water
Answer & solution

Answer: (b) Water and salt

Correct Answer: Option 2 — Water and salt
The reaction between an acid and a base which gives a salt and water is called a neutralisation reaction. Here hydrochloric acid (HCl), a strong acid, reacts with sodium hydroxide (NaOH), a strong base, to give the salt sodium chloride (NaCl) and water.
⚗️ The Reaction
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• In ionic form the essence of the reaction is H⁺ + OH⁻ → H₂O.
• Na⁺ pairs with Cl⁻ to give the salt, so it is also a double displacement reaction.
• The reaction is exothermic — the mixture becomes noticeably warm.
• NaCl is the salt of a strong acid and a strong base, so its solution is neutral, pH = 7.
The general form worth memorising is Acid + Base → Salt + Water.
❌ Why the other options are incorrect
✗ Only water — water is only one of the two products; the metal of the base and the negative ion of the acid must also combine, forming the salt.
✗ Only salt — the H⁺ of the acid and the OH⁻ of the base necessarily combine, and water is always produced in a neutralisation.
✗ Carbon dioxide and water — CO₂ appears when an acid reacts with a carbonate or a bicarbonate, e.g. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. NaOH contains no carbon at all.
📌 SSC & Railway Exam Facts
• Acid + Base → Salt + Water (neutralisation); Acid + Metal → Salt + H₂; Acid + Metal carbonate → Salt + H₂O + CO₂.
• NaOH is caustic soda, KOH is caustic potash, Ca(OH)₂ is slaked lime.
• Antacids such as milk of magnesia, Mg(OH)₂ work by neutralising excess stomach HCl.
• Baking soda is used on a bee sting (acidic) and vinegar on a wasp sting (alkaline).
• NaCl solution is neutral (pH 7), Na₂CO₃ solution is basic, NH₄Cl solution is acidic.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Properties of bases and alkalis

Q135. Which of the following substances can act as a source of OH⁻ ions when dissolved in water?

  1. (a) Sodium hydroxide
  2. (b) Carbon dioxide
  3. (c) Hydrochloric acid
  4. (d) Nitric acid
Answer & solution

Answer: (a) Sodium hydroxide

Correct Answer: Option 1 — Sodium hydroxide
According to the Arrhenius concept, a base is a substance that produces hydroxide ions, OH⁻, when dissolved in water, while an acid is a substance that produces hydrogen ions, H⁺ (as H₃O⁺), in water.
Sodium hydroxide (NaOH) is a strong base; it dissolves in water and ionises almost completely, flooding the solution with OH⁻ ions.
⚗️ The Reaction
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
• Because ionisation is nearly complete, NaOH is a strong alkali — a base that is soluble in water.
• Its solution turns red litmus blue, phenolphthalein pink and has a pH well above 7 (about 14 for 1 M NaOH).
• Dissolving NaOH in water is highly exothermic, so the acid/base must always be added to water, never the reverse.
❌ Why the other options are incorrect
✗ Carbon dioxide — an acidic oxide; dissolving in water it gives carbonic acid, CO₂ + H₂O → H₂CO₃, which releases H⁺, not OH⁻.
✗ Hydrochloric acid — a strong acid, ionising as HCl → H⁺ + Cl⁻; it is the acid present in gastric juice.
✗ Nitric acid — also a strong acid, HNO₃ → H⁺ + NO₃⁻, used in making fertilisers and explosives.
📌 SSC & Railway Exam Facts
• Common bases: NaOH (caustic soda), KOH (caustic potash), Ca(OH)₂ (slaked lime), NH₄OH, Mg(OH)₂ (milk of magnesia).
• All alkalis are bases, but all bases are not alkalis — only water-soluble bases are alkalis.
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic; the pH scale was given by S. P. L. Sørensen.
• Turmeric and china rose are natural indicators; onion and vanilla are olfactory indicators.
• Ca(OH)₂ + CO₂ → CaCO₃ + H₂O — the lime-water test that turns lime water milky.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · pH scale and its importance

Q136. Which of the following ion concentrations is measured by the pH scale?

  1. (a) Na⁺ ion concentration
  2. (b) Cl⁻ ion concentration
  3. (c) H⁺ ion concentration
  4. (d) F⁻ ion concentration
Answer & solution

Answer: (c) H⁺ ion concentration

Correct Answer: Option 3 — H⁺ ion concentration
The pH scale measures the concentration of hydrogen ions (H⁺) present in a solution. In fact the "p" in pH stands for the German "potenz", meaning power, so pH literally means the "power of hydrogen".
Mathematically, pH = −log[H⁺], where [H⁺] is the hydrogen-ion concentration in mol/L. The scale runs from 0 to 14:
• pH < 7 → acidic, because the H⁺ concentration is high (a lower pH means a stronger acid).
• pH = 7 → neutral, as in pure water, where [H⁺] = [OH⁻].
• pH > 7 → basic/alkaline, because the OH⁻ concentration exceeds the H⁺ concentration.
The scale was introduced by the Danish chemist S. P. L. Sørensen in 1909. A change of one pH unit means a tenfold change in hydrogen-ion concentration.
❌ Why the other options are incorrect
✗ Na⁺ ion concentration — sodium ions are spectator ions; they do not decide whether a solution is acidic or basic.
✗ Cl⁻ ion concentration — chloride ions likewise take no part; a 1 M NaCl solution is neutral even though it is full of Cl⁻.
✗ F⁻ ion concentration — fluoride content matters for dental health and fluorosis, but it is not what the pH scale measures.
📌 SSC & Railway Exam Facts
• pH = −log[H⁺]; pH + pOH = 14 at 25 °C.
• Human blood pH = 7.35–7.45; saliva ≈ 6.8; gastric juice ≈ 1.2–3.0; pure water = 7; milk ≈ 6.6.
• Rain below pH 5.6 is acid rain, caused by SO₂ and NO₂.
• Tooth decay begins when the mouth pH falls below 5.5; toothpastes are basic to neutralise this.
• Plants and crops grow best in a narrow pH range of soil, close to neutral.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 1 · pH scale and its importance

Q137. Select the option that completes the given sentence correctly.
The pH scale is used to measure the level of ______ of a solution.

  1. (a) density
  2. (b) pollution
  3. (c) acidity or basicity
  4. (d) volume
Answer & solution

Answer: (c) acidity or basicity

Correct Answer: Option 3 — acidity or basicity
The pH scale measures the concentration of hydrogen ions (H⁺) in a solution and therefore tells us how acidic or how basic that solution is. The letter p stands for the German word "potenz", meaning power, so pH = power of hydrogen. The scale was given by S. P. L. Sørensen and runs from 0 to 14.
• pH < 7 → acidic — the more H⁺ ions, the lower the pH.
• pH = 7 → neutral — as in pure water.
• pH > 7 → basic (alkaline) — more OH⁻ ions.
A universal indicator shows the whole range by colour: red for strongly acidic, green for neutral and violet/blue for strongly basic.
❌ Why the other options are incorrect
✗ density — density is mass per unit volume, measured with a hydrometer, and has nothing to do with H⁺ ions.
✗ pollution — pollution levels are measured by parameters such as AQI, BOD or TDS, not by pH alone.
✗ volume — volume is measured with a measuring cylinder, pipette or burette; it is a physical quantity, not a chemical property.
📌 SSC & Railway Exam Facts
• pH scale 0–14; pH 7 = neutral; term coined by Sørensen; p = potenz (power).
• Gastric juice pH ≈ 1.2, lemon juice ≈ 2.2, pure water = 7, blood ≈ 7.4, milk of magnesia ≈ 10.
• Acid rain has pH below 5.6 and damages crops and monuments.
• Our mouth must stay above pH 5.5 or tooth enamel (calcium phosphate) starts to decay.
• A bee sting is acidic — relieved by baking soda; a wasp sting is basic — relieved by vinegar.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Neutralisation reaction

Q138. The chemical reaction between an acid and base is termed as:

  1. (a) condensation reaction
  2. (b) endothermic reaction
  3. (c) decomposition reaction
  4. (d) neutralisation reaction
Answer & solution

Answer: (d) neutralisation reaction

Correct Answer: Option 4 — neutralisation reaction
The reaction between an acid and a base in which they cancel each other's effect and give a salt and water is called a neutralisation reaction.
⚗️ The Reaction
Acid + Base → Salt + Water
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• In essence, the H⁺ ion of the acid joins the OH⁻ ion of the base to form water: H⁺ + OH⁻ → H₂O.
• Heat is always given out, so neutralisation is an exothermic reaction.
• At the end point the solution becomes neutral, pH ≈ 7.
Everyday uses: an antacid (Mg(OH)₂, milk of magnesia) neutralising excess stomach acid, baking soda on a bee sting, and quick lime or slaked lime added to acidic soil.
❌ Why the other options are incorrect
✗ condensation reaction — an organic reaction in which two molecules join with the elimination of a small molecule like water (esterification); it is not the acid–base reaction meant here.
✗ endothermic reaction — one that absorbs heat; neutralisation actually releases heat, so it is exothermic.
✗ decomposition reaction — a single compound breaks into simpler substances (CaCO₃ → CaO + CO₂); here two compounds react to give two products.
📌 SSC & Railway Exam Facts
• Acid + Base → Salt + Water (neutralisation, exothermic).
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic; pH scale given by Sørensen.
• Litmus: acid turns blue litmus red; base turns red litmus blue.
• Phenolphthalein — colourless in acid, pink in base; methyl orange — red in acid, yellow in base.
• Stomach acid is HCl; antacids such as milk of magnesia, Mg(OH)₂ neutralise it.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q139. Select the correct chemical reaction that represents the chlor-alkali process.

  1. (a) 2KCl(aq) + 2H₂O(l) → 2KOH(aq) + Cl₂(g) + H₂(g)
  2. (b) 2MgCl₂(aq) + 2H₂O(l) → 2Mg(OH)₂(aq) + Cl₂(g) + H₂(g)
  3. (c) 2CaCl₂(aq) + 2H₂O(l) → 2Ca(OH)₂(aq) + Cl₂(g) + H₂(g)
  4. (d) 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
Answer & solution

Answer: (d) 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Correct Answer: Option 4 — 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
The chlor-alkali process is the electrolysis of a concentrated aqueous solution of sodium chloride, called brine. The name comes from its two main products — "chlor" for chlorine and "alkali" for sodium hydroxide.
⚗️ The Reaction
2NaCl(aq) + 2H₂O(l) → (electricity) 2NaOH(aq) + Cl₂(g) + H₂(g)
• Chlorine gas (Cl₂) is liberated at the anode.
• Hydrogen gas (H₂) is liberated at the cathode.
• Sodium hydroxide (NaOH) solution is left near the cathode.
• Count the atoms: 2 Na, 2 Cl, 4 H, 2 O on both sides — the equation is balanced.
Uses: NaOH for soaps, detergents and paper; Cl₂ for water treatment, PVC and bleaching powder; H₂ for fuels, margarine and ammonia.
❌ Why the other options are incorrect
✗ 2KCl(aq) + 2H₂O(l) → 2KOH(aq) + Cl₂(g) + H₂(g) — chemically possible, but the chlor-alkali industry uses common salt (NaCl) brine, not potassium chloride; the process is defined for NaCl.
✗ 2MgCl₂(aq) + 2H₂O(l) → 2Mg(OH)₂(aq) + Cl₂(g) + H₂(g) — wrong on two counts: Mg(OH)₂ is insoluble, so it cannot be (aq), and the equation is not balanced (4 Cl on the left, only 2 on the right).
✗ 2CaCl₂(aq) + 2H₂O(l) → 2Ca(OH)₂(aq) + Cl₂(g) + H₂(g) — again unbalanced in chlorine (4 on the left, 2 on the right), and Ca(OH)₂ is only sparingly soluble.
📌 SSC & Railway Exam Facts
• Chlor-alkali = electrolysis of brine; Cl₂ at anode, H₂ at cathode, NaOH near cathode.
• Cl₂ + dry slaked lime Ca(OH)₂ → bleaching powder, CaOCl₂.
• NaOH is caustic soda; Na₂CO₃·10H₂O is washing soda; NaHCO₃ is baking soda.
• NaCl is the raw material for caustic soda, washing soda, baking soda and bleaching powder.
• Baking soda + acid → CO₂, which makes cakes rise; NaHCO₃ is also used in soda-acid fire extinguishers.
RRB Technician Grade III 2024-25 – 20 December 2024 Shift 2 · Indicators (natural, synthetic, olfactory)

Q140. Which of the following is the olfactory indicator to check the acidic or basic nature of the substance?

  1. (a) Red litmus
  2. (b) Vanilla essence
  3. (c) Phenolphthalein
  4. (d) Methyl orange
Answer & solution

Answer: (b) Vanilla essence

Correct Answer: Option 2 — Vanilla essence
An olfactory indicator is a substance whose smell (odour) changes in acidic or basic medium — it is detected with the nose, not the eye. The three commonly quoted olfactory indicators are vanilla essence, onion and clove oil.
Vanilla essence keeps its characteristic sweet smell in an acidic solution, but in a basic solution (for example NaOH) its smell disappears. The same happens with onion — its pungent smell vanishes in a base and survives in an acid. This is why olfactory indicators are especially useful for visually impaired students working in a laboratory.
❌ Why the other options are incorrect
✗ Red litmus — a natural visual indicator obtained from lichens; red litmus turns blue in a base and stays red in an acid. It shows a colour change, not a change of smell.
✗ Phenolphthalein — a synthetic visual indicator: colourless in acid and pink in base.
✗ Methyl orange — another synthetic visual indicator: red in acid and yellow in base (orange when neutral).
📌 SSC & Railway Exam Facts
• Olfactory indicators: onion, vanilla essence and clove oil.
• Natural indicators: litmus (from lichens), turmeric, red cabbage and china rose (hibiscus) petals.
• Litmus: acid → red, base → blue; turmeric: base → red (why a curry stain on cloth turns red with soap).
• Phenolphthalein: acid → colourless, base → pink. Methyl orange: acid → red, base → yellow.
• Universal indicator gives the pH on a 0–14 scale: pH < 7 acidic, pH = 7 neutral, pH > 7 basic.
RRB Technician Grade III 2024-25 – 20 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q141. What is the chemical composition of washing soda?

  1. (a) Na₂CO₃·10H₂O
  2. (b) Al₂(SO₄)₃·10H₂O
  3. (c) Na₂SO₄·10H₂O
  4. (d) Al₂(CO₃)₃·10H₂O
Answer & solution

Answer: (a) Na₂CO₃·10H₂O

Correct Answer: Option 1 — Na₂CO₃·10H₂O
Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O — sodium carbonate crystallised together with 10 molecules of water of crystallisation. It is a white crystalline solid, basic in nature (pH about 11), and is obtained by recrystallising soda ash (anhydrous Na₂CO₃), which itself is manufactured by the Solvay (ammonia–soda) process.
The middle dot in the formula is important: the 10H₂O is water of crystallisation, chemically bound inside the crystal, not surface moisture. On heating, washing soda loses this water and becomes white powdery anhydrous Na₂CO₃.
Uses: softening hard water, washing clothes, in the glass, soap and paper industries, and as a common laboratory reagent.
❌ Why the other options are incorrect
✗ Al₂(SO₄)₃·10H₂O — a hydrate of aluminium sulphate; aluminium sulphate and potash alum are used for purifying water and to stop bleeding, not as washing soda.
✗ Na₂SO₄·10H₂O — this is Glauber's salt (sodium sulphate decahydrate), used as a laxative and in the glass industry; here the radical is sulphate, not carbonate.
✗ Al₂(CO₃)₃·10H₂O — aluminium carbonate is not even a stable compound; this option is fabricated only to mislead.
📌 SSC & Railway Exam Facts
• Washing soda = Na₂CO₃·10H₂O; soda ash = anhydrous Na₂CO₃; baking soda = NaHCO₃.
• Baking powder = baking soda + a mild edible acid (tartaric acid); it releases CO₂ and makes cakes rise.
• Bleaching powder = CaOCl₂, made by passing Cl₂ over dry slaked lime, Ca(OH)₂.
• Plaster of Paris = CaSO₄·½H₂O, obtained by heating gypsum (CaSO₄·2H₂O) to about 100 °C.
• Blue vitriol = CuSO₄·5H₂O, green vitriol = FeSO₄·7H₂O, Glauber's salt = Na₂SO₄·10H₂O.
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · pH scale and its importance

Q142. A solution has a pH of 12. Which ion concentration is higher in this solution?

  1. (a) More number S^(2-) ions and more Cl^(-1)
  2. (b) Only H^(+)
  3. (c) Only OH^(−)
  4. (d) Both H^(+ )and OH^(−) are equal
Answer & solution

Answer: (c) Only OH^(−)

Answer: Option 3 — Only OH^(−)
pH 12 is above 7, so the solution is basic — the hydroxide ion (OH^(−)) concentration is higher.
(H^(+) ions are still present, but only about 10^(−12) mol/L against 10^(−2) mol/L of OH^(−); "only" means only OH^(−) is in excess.)
Revision note — pH scale
• pH measures the hydrogen ion concentration; scale runs from 0 to 14.
• pH < 7: acidic (more H^(+)); pH = 7: neutral (pure water); pH > 7: basic (more OH^(−)).
• The lower the pH, the stronger the acid; the higher the pH, the stronger the base.
• At 25 °C, pH + pOH = 14 — pH 12 means pOH 2.
• Everyday values: gastric juice ≈ 1–2, lemon juice ≈ 2.5, pure water 7, blood ≈ 7.4, milk of magnesia ≈ 10, NaOH solution ≈ 14.
• Universal indicator shows pH by colour — red (strong acid) to violet (strong base).
Also asked from this topic: the substance that is neutral on the pH scale (pure water).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · pH scale and its importance

Q143. If a substance has a pH of 11, which of the following statements is true?

  1. (a) It is acidic and does not change the colour of the litmus paper.
  2. (b) It is neutral and does not change the litmus colour.
  3. (c) It is basic and can turn red litmus paper blue.
  4. (d) It is acidic and can turn blue litmus paper red.
Answer & solution

Answer: (c) It is basic and can turn red litmus paper blue.

Answer: Option 3 — It is basic and can turn red litmus paper blue.
A pH above 7 means the solution is basic; pH 11 is a fairly strong base.
Bases turn red litmus blue.
Revision note — The pH scale (Class 10)
• pH runs from 0 to 14: below 7 acidic, 7 neutral, above 7 basic.
• pH = −log[H⁺] — more H⁺ ions, lower pH; each unit step is a 10-times change in H⁺.
• Acids turn blue litmus red; bases turn red litmus blue.
• Phenolphthalein is colourless in acid and pink in base; methyl orange is red in acid and yellow in base.
• Common values: gastric juice about 1–2, lemon juice about 2.2, pure water 7, blood 7.4, milk of magnesia about 10, NaOH solution about 14.
• Tooth decay starts when mouth pH falls below 5.5.
Also asked from this topic: the neutral substance on the pH scale (pure water); what more H⁺ ions do to pH (lower it); the colour of phenolphthalein in a base (pink).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 1 · Properties of acids

Q144. Which ion is responsible for the acidic properties of acids?

  1. (a) Cl^(−)
  2. (b) OH^(−)
  3. (c) H^(+)
  4. (d) Na^(+)
Answer & solution

Answer: (c) H^(+)

Answer: Option 3 — H^(+)
Acids release hydrogen ions (H^(+)) when dissolved in water.
These H^(+) ions give acids their sour taste, turn blue litmus red and react with metals and bases.
Revision note — Acids and their ions
• H^(+) cannot exist alone; it joins water to form the hydronium ion (H_(3)O^(+)).
• Acids show acidic behaviour only in water — dry HCl gas does not change dry litmus.
• Bases/alkalis give hydroxide ions (OH^(−)) in water.
• Neutralisation: H^(+) + OH^(−) → H_(2)O.
• More H^(+) ⇒ lower pH; acids have pH < 7.
• Strong acids (HCl, H_(2)SO_(4), HNO_(3)) ionise fully; weak acids (acetic acid) ionise partly.
Do not mix these up
• H^(+) vs OH^(−) — H^(+) makes a solution acidic; OH^(−) makes it basic.
Also asked from this topic: the ion H^(+) is always shown as (hydronium ion, H_(3)O^(+)); why an acid shows acidic behaviour only in water (H^(+) ions form only on dissolving).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 2 · Nature of salts (acidic, basic, neutral)

Q145. When sodium carbonate (Na₂CO₃) is dissolved in water, the resulting solution becomes:

  1. (a) highly reactive with sodium chloride
  2. (b) basic
  3. (c) acidic
  4. (d) neutral
Answer & solution

Answer: (b) basic

Answer: Option 2 — basic
Sodium carbonate is the salt of a strong base (NaOH) and a weak acid (H₂CO₃).
In water it hydrolyses and gives OH⁻ ions, so the solution has pH above 7 — it is basic.
Revision note — Nature of salt solutions
• Strong acid + strong base → neutral salt, pH 7; e.g. NaCl, Na₂SO₄, KNO₃.
• Strong acid + weak base → acidic salt, pH below 7; e.g. NH₄Cl, CuSO₄.
• Weak acid + strong base → basic salt, pH above 7; e.g. Na₂CO₃, NaHCO₃, CH₃COONa.
• Washing soda = Na₂CO₃·10H₂O (10 water of crystallisation); used in glass, soap, paper industries and to remove permanent hardness of water.
• Baking soda = NaHCO₃ — mildly basic, used as an antacid and in soda-acid fire extinguishers.
• Na₂CO₃ turns red litmus blue and phenolphthalein pink.
Do not mix these up
• Washing soda vs baking soda — Na₂CO₃·10H₂O is washing soda; NaHCO₃ is baking soda.
Also asked from this topic: the nature of NaCl solution (neutral); the salt among NH₄Cl, Na₂CO₃ and sodium acetate that gives an acidic solution (NH₄Cl); the salt type that gives pH 9 (strong base + weak acid).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 2 · pH scale and its importance

Q146. When a solution contains an equal number of H^(+) and OH^(−) ions, then the pH of the solution is:

  1. (a) pH &lt; 3
  2. (b) pH &gt; 12
  3. (c) pH = 7
  4. (d) pH &gt; 3 but &lt; 5
Answer & solution

Answer: (c) pH = 7

Answer: Option 3 — pH = 7
When [H^(+)] = [OH^(−)], the solution is neutral.
At 25 °C, each concentration is 10^(−7) mol/L, so pH = 7.
Revision note — The pH scale
• pH = −log[H^(+)]; scale runs from 0 to 14 (Sørensen).
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic.
• Each unit of pH is a 10-fold change in [H^(+)].
• Common values: gastric juice about 1.2, lemon juice about 2.2, pure water 7, blood about 7.4, milk of magnesia about 10.
• Tooth decay starts when mouth pH falls below 5.5.
• Universal indicator shows different colours at different pH values.
Also asked from this topic: the substance that is neutral on the pH scale (pure water); the pH effect of a higher H^(+) concentration (lower pH); the nature of the salt of a strong acid and strong base (neutral).
Neutral means equal H^(+) and OH^(−), not zero of either.
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q147. Which reactants are used to prepare bleaching powder?

  1. (a) Calcium hydroxide and hydrochloric acid
  2. (b) Chlorine gas and dry calcium hydroxide
  3. (c) Lime and sulphuric acid
  4. (d) Chlorine and water
Answer & solution

Answer: (b) Chlorine gas and dry calcium hydroxide

Answer: Option 2 — Chlorine gas and dry calcium hydroxide
Bleaching powder is made by passing chlorine gas over dry slaked lime (calcium hydroxide).
Ca(OH)_(2) + Cl_(2) → CaOCl_(2) + H_(2)O
Revision note — Bleaching powder
• Chemical name calcium oxychloride; formula CaOCl_(2).
• The chlorine comes from the chlor-alkali process — electrolysis of brine gives NaOH, Cl_(2) (at the anode) and H_(2) (at the cathode).
• It bleaches by releasing chlorine, which acts as an oxidising agent.
• Uses: bleaching cotton and linen (textiles), wood pulp (paper), washed clothes (laundry).
• Also used to disinfect drinking water and as an oxidising agent in chemical industries.
Do not mix these up
• Bleaching powder CaOCl_(2) · baking soda NaHCO_(3) · washing soda Na_(2)CO_(3)·10H_(2)O · plaster of Paris CaSO_(4)·½H_(2)O.
Also asked from this topic: the gas that makes it bleach (chlorine); a use of bleaching powder besides bleaching (disinfecting drinking water); the name of the electrolysis of brine (chlor-alkali process).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q148. Washing soda is represented by Na_(2)CO_(3).xH_(2)O. The value of 'x' is equal to:

  1. (a) 10
  2. (b) 6
  3. (c) 8
  4. (d) 2
Answer & solution

Answer: (a) 10

Answer: Option 1 — 10
Washing soda is sodium carbonate decahydrate, Na_(2)CO_(3)·10H_(2)O.
Deca = ten, so each formula unit carries 10 molecules of water of crystallisation.
Revision note — Common salts and water of crystallisation
• Washing soda: Na_(2)CO_(3)·10H_(2)O — made by recrystallising sodium carbonate; heating gives soda ash (anhydrous Na_(2)CO_(3)).
• Blue vitriol (copper sulphate): CuSO_(4)·5H_(2)O — turns white on heating as it loses water.
• Gypsum: CaSO_(4)·2H_(2)O; Plaster of Paris: CaSO_(4)·½H_(2)O.
• Baking soda: NaHCO_(3) — no water of crystallisation.
• Uses of washing soda: glass, soap and paper industries, making borax, household cleaning, removing permanent hardness of water.
Do not mix these up
• Washing soda vs baking soda — Na_(2)CO_(3)·10H_(2)O (sodium carbonate) vs NaHCO_(3) (sodium hydrogencarbonate).
Also asked from this topic: industries that use washing soda (glass, soap and paper); the chemical name of baking soda (sodium bicarbonate); calcium sulphate hemihydrate (Plaster of Paris).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q149. Baking soda, commonly used in households, is also known by its chemical name:

  1. (a) sodium bicarbonate
  2. (b) magnesium hydroxide
  3. (c) sodium carbonate
  4. (d) calcium carbonate
Answer & solution

Answer: (a) sodium bicarbonate

Answer: Option 1 — sodium bicarbonate
Baking soda is sodium hydrogencarbonate (sodium bicarbonate), NaHCO_(3).
Revision note — Baking soda
• Made from NaCl + H_(2)O + CO_(2) + NH_(3) → NH_(4)Cl + NaHCO_(3).
• On heating: 2NaHCO_(3) → Na_(2)CO_(3) + H_(2)O + CO_(2) — the CO_(2) makes cakes rise.
• Baking powder = baking soda + a mild edible acid such as tartaric acid.
• It is a mild base — used as an antacid and in soda-acid fire extinguishers.
• Its solution is basic; it is not corrosive.
Do not mix these up
• Sodium bicarbonate vs sodium carbonate — baking soda is NaHCO_(3); Na_(2)CO_(3) is soda ash / washing soda.
• Magnesium hydroxide is milk of magnesia; calcium carbonate is limestone / chalk / marble.
Also asked from this topic: why baking soda is used in fire extinguishers (it releases carbon dioxide easily with acid); the gas needed with ammonia to make it (carbon dioxide).
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q150. The chemical name for 'Plaster of Paris' is:

  1. (a) calcium sulphate dihydrate
  2. (b) calcium chloride hexahydrate
  3. (c) calcium chloride semihydrate
  4. (d) calcium sulphate hemihydrate
Answer & solution

Answer: (d) calcium sulphate hemihydrate

Answer: Option 4 — calcium sulphate hemihydrate
Plaster of Paris is CaSO_(4)·½H_(2)O — two formula units of calcium sulphate share one water molecule.
"Hemi" means half.
Revision note — Plaster of Paris and gypsum (Class 10)
• Gypsum = CaSO_(4)·2H_(2)O (calcium sulphate dihydrate).
• Heating gypsum at 373 K gives Plaster of Paris: CaSO_(4)·2H_(2)O → CaSO_(4)·½H_(2)O + 1½H_(2)O.
• Mixed with water, Plaster of Paris sets into a hard solid mass — it changes back to gypsum.
• Uses: plaster for supporting fractured bones, toys, decoration material, making surfaces smooth.
• It is stored in moisture-proof containers, since moisture makes it set.
Do not mix these up
• Hemihydrate vs dihydrate — Plaster of Paris is ½H_(2)O; gypsum is 2H_(2)O.
Also asked from this topic: the process by which Plaster of Paris hardens with water (rehydration to form gypsum); why it is used for casts on fractured bones (sets quickly into a hard mass).
Option 1 (dihydrate) is gypsum, the raw material — not Plaster of Paris.
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 3 · Neutralisation reaction

Q151. Neutralisation reaction is a type of:

  1. (a) decomposition reaction
  2. (b) displacement reaction
  3. (c) double displacement reaction
  4. (d) combination reaction
Answer & solution

Answer: (c) double displacement reaction

Answer: Option 3 — double displacement reaction
In neutralisation the acid and the base exchange ions: NaOH + HCl → NaCl + H_(2)O.
Na⁺ pairs with Cl⁻ and H⁺ pairs with OH⁻ — an exchange of partners is a double displacement.
Revision note — Neutralisation (Class 10)
• Acid + base → salt + water; the reaction is exothermic (gives out heat).
• The core step is H⁺ + OH⁻ → H_(2)O.
• Double displacement: two compounds exchange ions (AB + CD → AD + CB).
• A double displacement that forms an insoluble solid is a precipitation reaction (BaCl_(2) + Na_(2)SO_(4) → BaSO_(4)↓ + 2NaCl).
• Everyday uses: antacids (milk of magnesia) for acidity, baking soda on bee stings, quicklime / slaked lime for acidic soil.
Do not mix these up
• Displacement vs double displacement — displacement: one element pushes another out (Zn + CuSO_(4)); double displacement: two compounds swap ions.
Also asked from this topic: the products when an acid reacts with a base (salt and water); what is added to acidic soil to neutralise it (quicklime).
Neutralisation forms two new compounds from two compounds — no single element is displaced.
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 3 · Water of crystallisation

Q152. When washing soda is exposed to hot atmosphere for a long time, it:

  1. (a) gains heat from the Sun and becomes dry
  2. (b) gains water molecules and dissolves in them
  3. (c) loses water molecules and becomes powdery
  4. (d) loses water molecules but remains in crystal form
Answer & solution

Answer: (c) loses water molecules and becomes powdery

Answer: Option 3 — loses water molecules and becomes powdery
Washing soda is Na_(2)CO_(3)·10H_(2)O.
In warm, dry air it loses 9 of its 10 water molecules and crumbles into a white powder (Na_(2)CO_(3)·H_(2)O).
This loss of water on exposure to air is called efflorescence.
Revision note — Water of crystallisation (Class 10)
• Water of crystallisation = the fixed number of water molecules in one formula unit of a salt crystal.
• Washing soda Na_(2)CO_(3)·10H_(2)O (10 H_(2)O); blue vitriol CuSO_(4)·5H_(2)O (5); gypsum CaSO_(4)·2H_(2)O (2); Plaster of Paris CaSO_(4)·½H_(2)O.
• Heating blue copper sulphate crystals drives off water — they turn white (anhydrous); adding water brings the blue back.
• The shape and colour of crystals depend on this water; losing it breaks the crystal.
• Washing soda is made by recrystallising sodium carbonate (from the Solvay process); uses: glass, soap, paper, removing permanent hardness of water.
Do not mix these up
• Efflorescence vs deliquescence — efflorescence: a salt loses water to air (washing soda); deliquescence: a salt absorbs water from air and dissolves (CaCl_(2), NaOH).
Also asked from this topic: the industries that use washing soda (glass, soap and paper).
A crystal cannot keep its crystal form after losing its water of crystallisation — option 4 is the trap.
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q153. The reactants that are used for the production of bleaching powder are:

  1. (a) CaOH + Cl_(2)
  2. (b) CaO + Cl
  3. (c) Ca(OH)_(2) + Cl_(2)
  4. (d) CaO + Cl_(2)
Answer & solution

Answer: (c) Ca(OH)_(2) + Cl_(2)

Answer: Option 3 — Ca(OH)_(2) + Cl_(2)
Bleaching powder is made by passing chlorine gas (Cl_(2)) over dry slaked lime, Ca(OH)_(2).
Ca(OH)_(2) + Cl_(2) → CaOCl_(2) + H_(2)O
Revision note — Bleaching powder
• Chemical name calcium oxychloride, formula CaOCl_(2).
• Chlorine comes from the chlor-alkali process (electrolysis of brine).
• Brine gives NaOH, Cl_(2) at the anode and H_(2) at the cathode.
• Its action comes from the chlorine it releases.
• Uses: bleaching cotton, linen and wood pulp; disinfecting drinking water; oxidising agent in industry.
• Other salts: baking soda NaHCO_(3), washing soda Na_(2)CO_(3)·10H_(2)O, Plaster of Paris CaSO_(4)·½H_(2)O.
Do not mix these up
• Slaked lime vs quicklime — slaked lime is Ca(OH)_(2); quicklime is CaO. Bleaching powder needs slaked lime.
Also asked from this topic: the gas that gives bleaching powder its action (chlorine); another use of bleaching powder (disinfecting water); the process that gives chlorine from brine (chlor-alkali process).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 1 · Neutralisation reaction

Q154. 20 ml of NaOH is neutralised by 10 ml of HNO_(3). How much NaOH will be required to neutralise 15 ml of HNO_(3)?

  1. (a) 20 ml
  2. (b) 30 ml
  3. (c) 40 ml
  4. (d) 25 ml
Answer & solution

Answer: (b) 30 ml

Rule — neutralisation of the same acid and base.
For complete neutralisation, moles of H^(+) from the acid = moles of OH^(−) from the base.
With the same two solutions, the volume of NaOH needed is directly proportional to the volume of HNO_(3).
Find the ratio from the given data.
20 ml NaOH ↔ 10 ml HNO_(3)
NaOH needed per ml of HNO_(3) = 20 ÷ 10 = 2 ml
Apply it to 15 ml HNO_(3).
NaOH = 15 × 2 = 30 ml
Answer: 30 ml
Revision note — Neutralisation
• Acid + base → salt + water; here HNO_(3) + NaOH → NaNO_(3) + H_(2)O.
• In solution, H^(+) of the acid and OH^(−) of the base combine to form water.
• Neutralisation is a type of double displacement reaction and is exothermic.
• HNO_(3) and HCl are monobasic acids; NaOH is a monoacidic base — they react 1 : 1 by moles.
• Everyday uses: antacid (milk of magnesia) for acidity, baking soda on a bee sting, lime for acidic soil.
Also asked from this topic: the products of an acid–base reaction (salt and water); the type of reaction HCl + NaOH → NaCl + H_(2)O (double displacement); the ions that form water (H^(+) and OH^(−)).
Half the acid needs half the base — the ratio 2 : 1 fixed in the first line carries straight to 15 ml.
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 1 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q155. Which of the following best describes a common industrial use of washing soda?

  1. (a) Cement production
  2. (b) Fertiliser manufacturing
  3. (c) Glass manufacturing
  4. (d) Vinegar production
Answer & solution

Answer: (c) Glass manufacturing

Answer: Option 3 — Glass manufacturing
Washing soda (sodium carbonate, Na_(2)CO_(3)·10H_(2)O) is a major raw material of the glass industry, together with sand and limestone.
It is also used in the soap and paper industries.
Revision note — Four common salts (Class 10 Acids, Bases and Salts)
• Washing soda — Na_(2)CO_(3)·10H_(2)O; made by recrystallising sodium carbonate.
• Uses of washing soda: glass, soap, paper, making borax, cleaning, removing permanent hardness.
• Baking soda — NaHCO_(3); uses: baking powder, antacid, soda-acid fire extinguisher.
• Bleaching powder — CaOCl_(2); made by passing chlorine over dry slaked lime; uses: bleaching, disinfecting drinking water, oxidising agent.
• Plaster of Paris — CaSO_(4)·½H_(2)O; made by heating gypsum at 373 K; uses: plaster for fractures, toys, decoration.
• Na_(2)CO_(3) solution is basic (salt of a strong base and a weak acid).
Do not mix these up
• Washing soda vs baking soda — washing soda is Na_(2)CO_(3)·10H_(2)O (cleaning, glass); baking soda is NaHCO_(3) (cooking, antacid).
Also asked from this topic: the industries that use sodium carbonate (glass, soap and paper); why baking soda suits fire extinguishers (releases CO_(2) easily with acid); a use of bleaching powder (oxidising agent).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q156. What is the molecular formula of washing soda?

  1. (a) NaHCO₃
  2. (b) Na₂CO₃
  3. (c) NaCl
  4. (d) Na₂CO₃·10H₂O
Answer & solution

Answer: (d) Na₂CO₃·10H₂O

Answer: Option 4 — Na_(2)CO_(3)·10H_(2)O
Washing soda is sodium carbonate decahydrate — sodium carbonate with 10 molecules of water of crystallisation.
Revision note — Common sodium and calcium salts
• Washing soda: Na_(2)CO_(3)·10H_(2)O; made by recrystallising soda ash (Na_(2)CO_(3)).
• Baking soda: NaHCO_(3) — sodium hydrogencarbonate; used in antacids and soda-acid fire extinguishers.
• Common salt: NaCl; its brine gives NaOH, Cl_(2) and H_(2) in the chlor-alkali process.
• Bleaching powder: CaOCl_(2); Plaster of Paris: CaSO_(4)·½H_(2)O; Gypsum: CaSO_(4)·2H_(2)O.
• Uses of washing soda: glass, soap and paper industries, making borax, household cleaning, removing permanent hardness of water.
Do not mix these up
• Na_(2)CO_(3) vs Na_(2)CO_(3)·10H_(2)O — the anhydrous form is soda ash; washing soda is the decahydrate.
• Washing soda vs baking soda — carbonate (Na_(2)CO_(3)) vs hydrogencarbonate (NaHCO_(3)).
Also asked from this topic: industries that use washing soda (glass, soap and paper); the gas needed with ammonia to prepare baking soda (carbon dioxide).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q157. What is the difference between baking powder and baking soda?

  1. (a) Baking powder is a mixture of baking soda and hydrochloric acid.
  2. (b) Baking soda is a mixture of baking powder and hydrochloric acid.
  3. (c) Baking soda is a mixture of baking powder and tartaric acid.
  4. (d) Baking powder is a mixture of baking soda and tartaric acid.
Answer & solution

Answer: (d) Baking powder is a mixture of baking soda and tartaric acid.

Answer: Option 4 — Baking powder is a mixture of baking soda and tartaric acid.
Baking soda is a single compound, sodium hydrogencarbonate (NaHCO_(3)).
Baking powder = baking soda + a mild edible acid such as tartaric acid.
Revision note — Baking soda and baking powder (Class 10)
• Baking soda is made from NaCl + H_(2)O + CO_(2) + NH_(3) → NH_(4)Cl + NaHCO_(3).
• On heating: 2NaHCO_(3) → Na_(2)CO_(3) + H_(2)O + CO_(2); the Na_(2)CO_(3) left behind makes cake bitter.
• In baking powder the tartaric acid neutralises this, so the cake is not bitter.
• With water or heat, baking powder gives off CO_(2), which makes bread and cake soft and spongy.
• Other uses of baking soda: antacid (neutralises excess stomach acid) and soda-acid fire extinguishers.
• Washing soda is Na_(2)CO_(3)·10H_(2)O — a different salt.
Do not mix these up
• Baking soda vs washing soda — NaHCO_(3) (cooking, antacid) vs Na_(2)CO_(3)·10H_(2)O (cleaning, removing permanent hardness).
Also asked from this topic: chemical name of baking soda (sodium hydrogencarbonate); why it is used in fire extinguishers (it readily releases CO_(2)); gas needed with ammonia to make it (carbon dioxide).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 2 · pH scale and its importance

Q158. What is the pH of a solution containing significant concentrations of H⁺ ions?

  1. (a) Less than 7
  2. (b) Cannot be determined
  3. (c) Greater than 7
  4. (d) 7
Answer & solution

Answer: (a) Less than 7

Answer: Option 1 — Less than 7
A high concentration of H⁺ ions makes a solution acidic, and acidic solutions have a pH below 7.
The more H⁺ ions, the lower the pH.
Revision note — The pH scale (Class 10)
• pH = "potenz" (power) of hydrogen; pH = −log[H⁺]; the scale runs from 0 to 14.
• pH < 7 → acidic; pH = 7 → neutral (pure water); pH > 7 → basic.
• Each unit change means a 10-fold change in H⁺ concentration.
• Universal indicator shows different colours at different pH values.
• Stomach juice (HCl) ≈ pH 2; blood ≈ 7.4; tooth decay starts when the mouth's pH falls below 5.5; acid rain has pH below 5.6.
• Plants and animals survive only in a narrow pH range (about 7.0 to 7.8 in our body).
Do not mix these up
• H⁺ vs OH⁻ — more H⁺ ions: acidic, pH down; more OH⁻ ions: basic, pH up.
Also asked from this topic: what a higher H⁺ concentration corresponds to (a lower pH); the neutral substance on the pH scale (pure water, pH 7); lowest H⁺ among seawater, blood, milk, vinegar (seawater).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q159. When Plaster of Paris is mixed with water, it undergoes which of the following changes?

  1. (a) It dissolves completely.
  2. (b) It loses all its water molecules.
  3. (c) It changes to gypsum.
  4. (d) It produces a gas.
Answer & solution

Answer: (c) It changes to gypsum.

Answer: Option 3 — It changes to gypsum.
Plaster of Paris takes up water again and sets into a hard solid mass of gypsum.
CaSO_(4)·½H_(2)O + 1½H_(2)O → CaSO_(4)·2H_(2)O
Revision note — Plaster of Paris
• Chemical name: calcium sulphate hemihydrate, CaSO_(4)·½H_(2)O — half a water molecule per formula unit.
• Made by heating gypsum (CaSO_(4)·2H_(2)O) at 373 K (100 °C).
• Heating above 373 K gives anhydrous CaSO_(4) (dead burnt plaster), which does not set with water.
• It must be stored in a moisture-proof container, or it sets before use.
• Uses: plaster for fractured bones, toys, decorative materials, and making surfaces smooth.
Do not mix these up
• Gypsum vs Plaster of Paris — gypsum has 2 water molecules; Plaster of Paris has ½.
Also asked from this topic: the chemical name and formula of Plaster of Paris (calcium sulphate hemihydrate, CaSO_(4)·½H_(2)O); why it is used for casts (it sets quickly into a hard mass).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q160. What is the role of chlorine gas in the preparation of bleaching powder?

  1. (a) It is used as a catalyst for the reaction.
  2. (b) It is used to neutralise calcium hydroxide.
  3. (c) It reacts with water to produce hydrogen peroxide.
  4. (d) It reacts with dry slaked lime to form bleaching powder.
Answer & solution

Answer: (d) It reacts with dry slaked lime to form bleaching powder.

Answer: Option 4 — It reacts with dry slaked lime to form bleaching powder.
Bleaching powder is made by the action of chlorine on dry slaked lime, Ca(OH)_(2).
Ca(OH)_(2) + Cl_(2) → CaOCl_(2) + H_(2)O
Revision note — Bleaching powder
• Chemical name: calcium oxychloride, CaOCl_(2).
• The chlorine comes from the chlor-alkali process — electrolysis of brine gives NaOH, Cl_(2) (at the anode) and H_(2) (at the cathode).
• Its bleaching and disinfecting action comes from the chlorine it releases.
• Uses: bleaching cotton and linen (textiles) and wood pulp (paper); bleaching washed clothes in laundry.
• Also an oxidising agent in chemical industries and a disinfectant for drinking water.
Also asked from this topic: the gas released by bleaching powder that bleaches and disinfects (chlorine); another use of bleaching powder (disinfecting drinking water).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 3 · pH scale and its importance

Q161. As the concentration of hydronium ions increases, the pH value _______.

  1. (a) decreases
  2. (b) decreases and then increases
  3. (c) remains the same
  4. (d) increases
Answer & solution

Answer: (a) decreases

Answer: Option 1 — decreases
pH measures the hydronium ion (H_(3)O^(+)) concentration on a reverse scale.
The higher the H_(3)O^(+) concentration, the more acidic the solution and the lower its pH.
Revision note — The pH scale
• pH — 'p' from German potenz (power); the scale runs from 0 (very acidic) to 14 (very alkaline).
• pH 7 = neutral (pure water); below 7 = acidic; above 7 = basic.
• pH = −log[H^(+)] — each fall of 1 in pH means 10 times more H^(+) ions.
• In water, H^(+) cannot exist alone; it joins water: H^(+) + H_(2)O → H_(3)O^(+).
• Common values: gastric juice ~1–2, lemon juice ~2.5, blood ~7.4, milk of magnesia ~10.
• Tooth decay starts when the mouth pH falls below 5.5; rain with pH below 5.6 is acid rain.
Also asked from this topic: what a higher hydrogen ion concentration gives (a lower pH); the ion formed when H^(+) joins water (hydronium ion); the pH of soap solution shows it is (basic, about 9).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q162. Why is bleaching powder used for disinfecting drinking water?

  1. (a) It reduces the hardness of water.
  2. (b) It neutralises the water.
  3. (c) It releases chlorine, which kills germs.
  4. (d) It provides an acidic medium.
Answer & solution

Answer: (c) It releases chlorine, which kills germs.

Answer: Option 3 — It releases chlorine, which kills germs.
Bleaching powder, calcium oxychloride (CaOCl_(2)), gives off chlorine, a strong oxidising agent that kills bacteria and other germs in water.
Revision note — Bleaching powder
• Made by passing chlorine over dry slaked lime: Ca(OH)_(2) + Cl_(2) → CaOCl_(2) + H_(2)O.
• The chlorine comes from the chlor-alkali process (electrolysis of brine), which also gives NaOH and H_(2).
• Uses: bleaching cotton and linen in textiles, wood pulp in paper mills, washed clothes in laundry.
• Also an oxidising agent in chemical industries and used to make chloroform.
• Disinfecting drinking water — chlorine is the active agent.
Do not mix these up
• Bleaching powder vs washing soda — bleaching powder disinfects; washing soda (Na_(2)CO_(3)·10H_(2)O) removes permanent hardness.
Also asked from this topic: the gas that gives bleaching powder its bleaching and disinfecting action (chlorine); a use of bleaching powder (disinfecting drinking water; oxidising agent in chemical industries).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · pH scale and its importance

Q163. The dark blue colour in the pH scale refers to __________.

  1. (a) strong acidic solution
  2. (b) strong basic solution
  3. (c) weak basic solution
  4. (d) neutral solution
Answer & solution

Answer: (c) weak basic solution

Answer: Option 3 — weak basic solution
On the colour chart of the universal indicator, blue to dark blue covers roughly pH 8 to 11 — a weak base.
The strongest bases (pH 12–14) show violet/purple.
Revision note — pH scale and universal indicator
• pH = measure of hydrogen ion concentration; scale runs 0 to 14 ("p" from German potenz, power).
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic; more H⁺ ions ⇒ lower pH.
• Universal indicator colours: red (strong acid) → orange/yellow (weak acid) → green (neutral) → blue (weak base) → violet (strong base).
• Pure water 7; gastric juice about 1.2; lemon juice about 2.2; blood about 7.4; milk of magnesia about 10; NaOH solution about 14.
• Tooth decay starts when mouth pH falls below 5.5.
• Acid rain has pH below 5.6; plants grow best in soil of pH near neutral.
Do not mix these up
• Universal indicator vs litmus — litmus only tells acid (red) or base (blue); universal indicator shows how strong through a range of colours.
Also asked from this topic: the substance that is neutral on the pH scale (pure water); what a higher H⁺ concentration means for pH (lower pH value).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q164. Washing soda (Na_(2)CO_(3)·10H_(2)O) is obtained from which compound in the Solvay process?

  1. (a) Sodium sulphate
  2. (b) Sodium chloride
  3. (c) Sodium hydroxide
  4. (d) Sodium bicarbonate
Answer & solution

Answer: (d) Sodium bicarbonate

Answer: Option 4 — Sodium bicarbonate
In the Solvay process sodium bicarbonate (NaHCO_(3)) is formed first.
Heating it gives sodium carbonate (Na_(2)CO_(3)), which is recrystallised with water to give washing soda.
Revision note — Solvay (ammonia-soda) process and washing soda
• Raw materials: brine (NaCl), ammonia (NH_(3)), carbon dioxide (CO_(2)) and limestone (CaCO_(3)).
• NaCl + NH_(3) + CO_(2) + H_(2)O → NaHCO_(3) + NH_(4)Cl (sodium bicarbonate settles out).
• 2NaHCO_(3) → Na_(2)CO_(3) + H_(2)O + CO_(2) on heating; anhydrous Na_(2)CO_(3) is called soda ash.
• Na_(2)CO_(3) + 10H_(2)O → Na_(2)CO_(3)·10H_(2)O (washing soda) by recrystallisation.
• Ammonia is recovered from NH_(4)Cl with slaked lime and used again.
• Washing soda is used in glass, soap and paper industries, to make borax, and to remove permanent hardness of water.
• Baking soda (NaHCO_(3)) is used in baking powder, antacids and soda-acid fire extinguishers.
Do not mix these up
• Raw material vs precursor — NaCl is the starting material; washing soda is obtained directly from sodium bicarbonate.
• Baking soda vs washing soda — NaHCO_(3) vs Na_(2)CO_(3)·10H_(2)O (10 water molecules).
Also asked from this topic: the gas needed with ammonia and water to make baking soda (carbon dioxide); industries that use washing soda (glass, soap and paper).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · pH scale and its importance

Q165. What is the pH range of a strongly acidic solution?

  1. (a) 0-3
  2. (b) 7-8
  3. (c) 12-14
  4. (d) 10-11
Answer & solution

Answer: (a) 0-3

Answer: Option 1 — 0-3
The lower the pH, the more acidic the solution.
Values close to 0 (about 0–3) mean a strongly acidic solution with a very high H^(+) ion concentration.
Revision note — The pH scale
• pH = potenz (power) of hydrogen ion; the scale runs from 0 to 14.
• pH < 7 → acidic; pH = 7 → neutral (pure water); pH > 7 → basic.
• 0–3 strongly acidic; 11–14 strongly basic; values near 7 are weak.
• Higher [H^(+)] means lower pH; each unit of pH is a 10-fold change in H^(+).
• Common values: gastric juice ≈ 1.2, lemon juice ≈ 2.2, pure water 7, blood ≈ 7.4, milk of magnesia ≈ 10, NaOH solution ≈ 14.
• Tooth decay starts below pH 5.5; the stomach's HCl keeps it strongly acidic; acid rain has pH below 5.6.
• A universal indicator shows the pH by a range of colours.
Do not mix these up
• Strong acid vs concentrated acid — strong means it ionises fully (HCl); concentrated means much acid in little water.
Also asked from this topic: a higher H^(+) concentration means (lower pH); the substance that is neutral on the pH scale (pure water).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q166. What is the process by which sodium hydroxide is prepared?

  1. (a) Contact process
  2. (b) Solvay’s process
  3. (c) Haber’s process
  4. (d) Chlor-alkali process
Answer & solution

Answer: (d) Chlor-alkali process

Answer: Option 4 — Chlor-alkali process
Electricity is passed through brine (aqueous NaCl). It breaks down to give sodium hydroxide, chlorine and hydrogen.
The name comes from the products: chlor (chlorine) + alkali (NaOH).
Revision note — Chlor-alkali process
• Equation: 2NaCl(aq) + 2H_(2)O(l) → 2NaOH(aq) + Cl_(2)(g) + H_(2)(g)
• Chlorine at the anode; hydrogen at the cathode; NaOH forms near the cathode.
• Uses of Cl_(2): water treatment, swimming pools, PVC, disinfectants, CFCs, pesticides, and making bleaching powder.
• Uses of NaOH: soaps and detergents, paper making, artificial fibres, de-greasing metals.
• Uses of H_(2): fuels, hydrogenation of oils (margarine), ammonia for fertilisers.
• Cl_(2) and H_(2) together give HCl, used for cleaning steel and in medicines.
Do not mix these up
• Solvay vs chlor-alkali — the Solvay process makes sodium carbonate (washing soda base); the chlor-alkali process makes sodium hydroxide.
• Haber vs Contact — Haber makes ammonia; Contact makes sulphuric acid.
Also asked from this topic: the name of the process in which brine is electrolysed (chlor-alkali process); the gas released at the cathode (hydrogen); a use of chlorine gas (disinfectants).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · Neutralisation reaction

Q167. Which of the following reactions does NOT represent a neutralisation reaction?

  1. (a) CH₃COOH + NaOH → CH₃COONa + H₂O
  2. (b) Na₂CO₃ + HCl → NaCl + CO₂ + H₂O
  3. (c) HCl + NaOH → NaCl + H₂O
  4. (d) H₂SO₄ + Ca(OH)₂ → CaSO₄ + 2H₂O
Answer & solution

Answer: (b) Na₂CO₃ + HCl → NaCl + CO₂ + H₂O

Answer: Option 2 — Na₂CO₃ + HCl → NaCl + CO₂ + H₂O
A neutralisation reaction is acid + base → salt + water only.
Sodium carbonate is a metal carbonate, and its reaction with HCl also gives carbon dioxide gas — this is the acid + metal carbonate reaction.
Balanced: Na_(2)CO_(3) + 2HCl → 2NaCl + CO_(2) + H_(2)O.
Revision note — Reactions of acids
• Acid + base → salt + water — neutralisation (e.g. HCl + NaOH → NaCl + H_(2)O).
• The core of every neutralisation is H⁺ + OH⁻ → H_(2)O; the reaction is exothermic.
• Acid + metal carbonate / hydrogencarbonate → salt + CO_(2) + water; CO_(2) turns lime water milky.
• Acid + metal → salt + hydrogen gas (H_(2) burns with a pop sound).
• Acid + metal oxide → salt + water — so metal oxides are basic.
• A weak acid (CH_(3)COOH) or a sparingly soluble base (Ca(OH)_(2)) still gives salt + water — still neutralisation.
Do not mix these up
• Neutralisation vs acid–carbonate reaction — look for CO_(2) among the products; neutralisation gives only salt + water.
Also asked from this topic: the gas released when a dilute acid reacts with a metal carbonate (carbon dioxide); the observation when HCl is added to sodium hydrogen carbonate (effervescence).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · Neutralisation reaction

Q168. Which of the following is the product of the neutralisation of hydrochloric acid (HCl) with sodium hydroxide (NaOH)?

  1. (a) Sodium chloride and water
  2. (b) Sodium hydroxide and hydrochloric acid
  3. (c) Sodium hydroxide and water
  4. (d) Sodium chloride and hydrogen gas
Answer & solution

Answer: (a) Sodium chloride and water

Answer: Option 1 — Sodium chloride and water
Acid + base → salt + water.
HCl + NaOH → NaCl + H_(2)O — the salt formed is sodium chloride (common salt).
Revision note — Neutralisation
• H⁺ from the acid and OH⁻ from the base combine to form water: H⁺ + OH⁻ → H_(2)O.
• The remaining ions (Na⁺ and Cl⁻) form the salt.
• Neutralisation is exothermic — heat is released.
• It is also a double displacement reaction (ions are exchanged).
• A strong acid + strong base (HCl + NaOH) gives a neutral salt, pH 7.
• Everyday uses: antacids (milk of magnesia) for acidity, baking soda for ant stings, quicklime / slaked lime for acidic soil.
Also asked from this topic: the ions that form water when HCl reacts with NaOH (H⁺ from HCl and OH⁻ from NaOH); the type of reaction HCl + NaOH → NaCl + H_(2)O also is (double displacement).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 2 · pH scale and its importance

Q169. What is the pH value of pure water at 25°C?

  1. (a) 5
  2. (b) 3
  3. (c) 1
  4. (d) 7
Answer & solution

Answer: (d) 7

Answer: Option 4 — 7
Pure water has equal H⁺ and OH⁻ ions (10⁻⁷ mol/L each at 25°C), so it is neutral with pH 7.
Revision note — pH scale
• pH = potential of hydrogen, given by S.P.L. Sørensen (1909); pH = −log[H⁺].
• Scale runs 0 to 14: below 7 acidic, 7 neutral, above 7 basic.
• More H⁺ ⇒ lower pH; each step of 1 pH is a 10-fold change in H⁺.
• Common values: gastric juice ≈ 1–2, lemon juice ≈ 2.5, milk ≈ 6.6, blood ≈ 7.4, sea water ≈ 8, milk of magnesia ≈ 10.
• Tooth decay starts when mouth pH falls below 5.5; acid rain has pH below 5.6.
Do not mix these up
• Neutral vs pure — any neutral solution (like NaCl in water) also has pH 7; it need not be pure water.
Also asked from this topic: the substance that is neutral on the pH scale (pure water); what a higher H⁺ concentration means (a lower pH).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 2 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q170. Which ion reduces at the cathode during electrolysis of fresh brine solution?

  1. (a) OH⁻
  2. (b) H^(+)
  3. (c) Na^(+)
  4. (d) Cl⁻
Answer & solution

Answer: (b) H^(+)

Answer: Option 2 — H^(+)
In brine, water supplies H^(+) ions, which are reduced (gain electrons) more easily than Na^(+).
At the cathode: 2H^(+) + 2e⁻ → H_(2) — hydrogen gas is given off, and Na^(+) stays in solution with OH⁻ as NaOH.
Revision note — Chlor-alkali process
• Electrolysis of brine (aqueous NaCl): 2NaCl + 2H_(2)O → 2NaOH + Cl_(2) + H_(2).
• Anode (+): Cl⁻ is oxidised → chlorine gas.
• Cathode (−): H^(+) (from water) is reduced → hydrogen gas.
• NaOH forms near the cathode — hence "chlor-alkali" (chlorine + alkali).
• Uses: Cl_(2) → bleaching powder, PVC, disinfectants; H_(2) → fuel, ammonia, hydrogenation of oils; NaOH → soaps, paper, degreasing metals.
Do not mix these up
• Oxidation vs reduction — reduction (gain of electrons) happens at the cathode; oxidation at the anode.
• Brine vs molten NaCl — only molten NaCl gives sodium metal at the cathode; in brine, hydrogen wins.
Also asked from this topic: the product formed at the cathode during electrolysis of brine (hydrogen gas); the name of the process that makes NaOH from brine (chlor-alkali process).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 3 · Common salts - preparation and uses (baking soda, washing soda, bleaching powder, plaster of Paris)

Q171. Which of the following reactions leads to the production of bleaching powder?

  1. (a) CaO + H_(2)O → Ca(OH)_(2)
  2. (b) Ca(OH)_(2) + Cl_(2) → CaOCl_(2) + H_(2)O
  3. (c) Ca(OH)_(2) + CO_(2) → CaCO_(3) + H_(2)O
  4. (d) CaCO_(3) + HCl → CaCl_(2) + CO_(2) + H_(2)O
Answer & solution

Answer: (b) Ca(OH)_(2) + Cl_(2) → CaOCl_(2) + H_(2)O

Answer: Option 2 — Ca(OH)_(2) + Cl_(2) → CaOCl_(2) + H_(2)O
Bleaching powder is made by passing chlorine gas over dry slaked lime [Ca(OH)_(2)].
The product is calcium oxychloride, CaOCl_(2) — bleaching powder.
Revision note — Bleaching powder and related salts
• Chlorine for it comes from the chlor-alkali process (electrolysis of brine).
• Bleaching powder releases chlorine, which does the bleaching and disinfecting.
• Uses: bleaching cotton, linen, wood pulp; disinfecting drinking water; oxidising agent in industry.
• CaO + H_(2)O → Ca(OH)_(2) — quicklime to slaked lime (exothermic combination).
• Ca(OH)_(2) + CO_(2) → CaCO_(3) + H_(2)O — lime water turns milky; test for CO_(2).
• Baking soda NaHCO_(3); washing soda Na_(2)CO_(3)·10H_(2)O; plaster of Paris CaSO_(4)·½H_(2)O.
Also asked from this topic: the gas responsible for bleaching action (chlorine); a use of bleaching powder besides bleaching (disinfecting drinking water); a use in industry (oxidising agent).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 3 · pH scale and its importance

Q172. Which of the following statements is true about the pH of a neutral solution at 25°C?

  1. (a) The pH is not equal to 7.
  2. (b) The pH is equal to 7.
  3. (c) The pH is greater than 7.
  4. (d) The pH is less than 7.
Answer & solution

Answer: (b) The pH is equal to 7.

Answer: Option 2 — The pH is equal to 7.
In a neutral solution the concentrations of H⁺ and OH⁻ ions are equal.
At 25°C each is 10^(−7) mol/L, so pH = 7.
Revision note — The pH scale
• pH = "potenz" (power) of hydrogen; the scale runs from 0 to 14 (proposed by S.P.L. Sørensen).
• pH < 7 — acidic; pH = 7 — neutral; pH > 7 — basic (alkaline).
• Lower pH = more H⁺ ions = stronger acid; each step of 1 is a 10-fold change in H⁺ concentration.
• Common values: gastric juice ≈ 1.2, lemon juice ≈ 2.2, pure water = 7, blood ≈ 7.4, milk of magnesia ≈ 10, NaOH ≈ 14.
• Our body works in the narrow range 7.0 – 7.8; tooth decay starts when mouth pH falls below 5.5.
• Universal indicator shows the full pH range by colour; litmus only tells acid or base.
Do not mix these up
• "Neutral" vs "pH 7" — pH 7 means neutral only at 25°C; at higher temperature pure water is still neutral but its pH is slightly below 7.
Also asked from this topic: the substance that is neutral on the pH scale (pure water); the most basic of lemon juice, soap solution, water and vinegar (soap solution, pH 9).
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