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Chemical Reactions and Equations PYQs – Chemistry, SSC & Railway

131 Chemical Reactions and Equations (Chemistry) questions asked in recent SSC and Railway exams, newest first, with exam, date & shift, answers and solutions.

📚 131 questionsSSC 1 · Railway 130
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RRB Group D 2025-26 – 25 August 2026 Shift 3 · Decomposition reactions (thermal, electrolytic, photolytic)

Q1. A chemist keeps calcium carbonate, silver chloride, and lead nitrate exposed to sunlight. Which compound/s show/s a visible change?

  1. (a) Only lead nitrate
  2. (b) Only silver chloride
  3. (c) Only calcium carbonate
  4. (d) All three compounds
Answer & solution

Answer: (b) Only silver chloride

Correct Answer:
Correct Answer: Option 2 – Only silver chloride
Explanation:
Silver chloride is photosensitive — it decomposes on exposure to sunlight, turning grey due to formation of metallic silver (2AgCl → 2Ag + Cl2), a photochemical decomposition reaction. Calcium carbonate and lead nitrate do not show this visible change from sunlight exposure alone (lead nitrate needs heating to decompose).
Important Facts:
• This photodecomposition of silver salts is the basis of old black-and-white photographic film.
• Lead nitrate decomposes on heating (thermal decomposition), releasing brown NO2 fumes, not simply from sunlight.
• Calcium carbonate is thermally stable and unaffected by sunlight.
RRB Group D 2025-26 – 21 August 2026 Shift 1 · Displacement reactions

Q2. Which equation correctly represents a displacement reaction involving metals and their salt solutions?

  1. (a) Au + FeSO₄ → AuSO₄ + Fe
  2. (b) Cu + ZnSO₄ → CuSO₄ + Zn
  3. (c) Zn + CuSO₄ → ZnSO₄ + Cu
  4. (d) Ag + MgSO₄ → AgSO₄ + Mg
Answer & solution

Answer: (c) Zn + CuSO₄ → ZnSO₄ + Cu

Explanation:
In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Zinc is more reactive than copper (higher in the reactivity series), so Zn displaces Cu from copper sulphate solution: Zn + CuSO4 → ZnSO4 + Cu. The other options are incorrect because in each of them the metal on the reactant side is less reactive than the metal in the salt (e.g., Cu cannot displace Zn; Au cannot displace Fe; Ag cannot displace Mg).
Correct Answer: Option 3 – Zn + CuSO4 → ZnSO4 + Cu
RRB Group D 2025-26 – 20 August 2026 Shift 2 · Double displacement and precipitation reactions

Q3. Consider the reaction:Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)Which feature of this reaction justifies classifying it specifically as a precipitation reaction?

  1. (a) Reactants exist initially as aqueous solutions
  2. (b) Two ionic compounds exchange partners
  3. (c) Insoluble barium sulphate forms in solution
  4. (d) Sodium chloride remains dissolved in water
Answer & solution

Answer: (c) Insoluble barium sulphate forms in solution

Correct Answer:
Correct Answer: Option 3 – Insoluble barium sulphate forms in solution.
Explanation:
A precipitation reaction is defined specifically by the formation of an insoluble solid (precipitate) from a reaction between soluble reactants. Here, BaSO₄ is insoluble and separates out of solution, which is the defining feature that makes this a precipitation reaction (it is also a double displacement reaction, but that classification does not by itself require a precipitate).
Important Facts:
• Double displacement reactions involve exchange of ions/partners between two compounds; they are called precipitation reactions specifically when an insoluble product is formed.
RRB Group D 2025-26 – 20 August 2026 Shift 2 · Decomposition reactions (thermal, electrolytic, photolytic)

Q4. Calcium carbonate is heated to produce calcium oxide and carbon dioxide; this is a type of:

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

Answer: (d) decomposition reaction

Correct Answer:
Correct Answer: Option 4 – Decomposition reaction
Explanation:
CaCO₃ →(heat) CaO + CO₂ is a classic example of thermal decomposition, where a single compound breaks down into two or more simpler substances on heating.
Important Facts:
• This reaction is used industrially to produce quicklime (CaO) from limestone.
• Decomposition is the reverse of a combination reaction.
RRB Group D 2025-26 – 20 August 2026 Shift 3 · Displacement reactions

Q5. When zinc reacts with dilute HCl, the reaction can be classified as:

  1. (a) Double decomposition reaction
  2. (b) Displacement reaction
  3. (c) Neutralization reaction
  4. (d) Combination reaction
Answer & solution

Answer: (b) Displacement reaction

Correct Answer:
Option 2 – Displacement reaction
Explanation:
Zn + 2HCl → ZnCl₂ + H₂↑. Here, the more reactive metal zinc displaces (replaces) hydrogen from hydrochloric acid, making this adisplacement reaction.
RRB Group D 2025-26 – 20 August 2026 Shift 3 · Corrosion and rancidity

Q6. Which condition will slow down rancidity in oily food items ?

  1. (a) Exposure to sunlight
  2. (b) Storage at high temperature
  3. (c) Presence of moisture
  4. (d) Storage in airtight containers flushed with nitrogen
Answer & solution

Answer: (d) Storage in airtight containers flushed with nitrogen

Correct Answer:
Option 4 – Storage in airtight containers flushed with nitrogen
Explanation:
Rancidity is caused by the oxidation of fats/oils by atmospheric oxygen. Storing oily foods inairtight containers flushed with nitrogen(an inert gas) excludes oxygen and thereby slows down rancidity. Exposure to sunlight, high temperature, and moisture all accelerate rancidity.
RRB Group D 2025-26 – 19 August 2026 Shift 1 · Double displacement and precipitation reactions

Q7. Consider the following reactions:(I) Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)(II) AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)(III) CaCO₃(s) —Heat→ CaO(s) + CO₂(g)(IV) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)Which of the above reactions are precipitation reactions?

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

Answer: (b) Only I and II

Correct Answer:
Correct Answer: Option 2 – Only I and II
Explanation:
Aprecipitation reactionis one in which two soluble salts react to form aninsoluble solid (precipitate). Reaction I formsBaSO₄(insoluble) and Reaction II formsAgCl(insoluble) — both precipitation reactions. Reaction III is athermal decompositionreaction, and Reaction IV is adisplacement (redox)reaction, not classified as precipitation.
Important Facts:
• Precipitation reactions are a type of double displacement (metathesis) reaction.
• CaCO₃ → CaO + CO₂ on heating is a decomposition reaction.
• Fe + CuSO₄ → FeSO₄ + Cu is a single displacement reaction (Fe displaces Cu).
RRB Group D 2025-26 – 19 August 2026 Shift 2 · Oxidation, reduction and redox reactions

Q8. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which substance is oxidized?

  1. (a) HCl
  2. (b) MnCl₂
  3. (c) H₂O
  4. (d) MnO₂
Answer & solution

Answer: (a) HCl

Correct Answer:
Correct Answer: Option 1 – HCl
Explanation:
In MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂: Manganese goes from +4 (in MnO₂) to +2 (in MnCl₂) — areduction. Chlorine (in HCl) goes from -1 to 0 (in Cl₂) — anoxidation, meaning HCl loses electrons and isoxidized. MnO₂ acts as the oxidizing agent, and HCl acts as the reducing agent (and is itself oxidized).
Important Facts:
• This reaction is used in the laboratory preparation of chlorine gas.
• Oxidation = loss of electrons / increase in oxidation state; Reduction = gain of electrons / decrease in oxidation state.
RRB Group D 2025-26 – 19 August 2026 Shift 3 · Oxidation, reduction and redox reactions

Q9. A student observes the reaction between zinc and sulfur resulting in a compound. Which property of metals is demonstrated when zinc combines with a non-metal like sulfur?

  1. (a) Zinc acts as an oxidizing agent
  2. (b) Zinc acts as a solvent
  3. (c) Zinc acts as a reducing agent
  4. (d) Zinc acts as a catalyst
Answer & solution

Answer: (c) Zinc acts as a reducing agent

Correct Answer:
Option 3 – Zinc acts as a reducing agent
Explanation:
When zinc reacts with sulfur to form zinc sulfide (Zn + S → ZnS), zincloses electronsand gets oxidized, while sulfur gains electrons and gets reduced. A substance that gets oxidized (loses electrons) while causing another substance to be reduced is called areducing agent. This demonstrates the general property of metals to lose electrons and act as reducing agents.
Important Facts:
• Metals generally act as reducing agents because they tend to lose electrons.
• Non-metals generally act as oxidizing agents because they tend to gain electrons.
Correct Answer: Option 3 – Zinc acts as a reducing agent
RRB Group D 2025-26 – 19 August 2026 Shift 3 · Oxidation, reduction and redox reactions

Q10. Which observation best demonstrates that reduction has occurred in a chemical reaction?

  1. (a) Hydrogen gas is evolved after metal reacts with acid.
  2. (b) Oxygen is removed from a metal oxide during heating.
  3. (c) Oxygen combines with a metal to form its oxide layer.
  4. (d) Carbon dioxide is released after heating a carbonate salt.
Answer & solution

Answer: (b) Oxygen is removed from a metal oxide during heating.

Correct Answer:
Option 2 – Oxygen is removed from a metal oxide during heating
Explanation:
Reductionis defined as the removal of oxygen (or gain of hydrogen/electrons) from a substance. When oxygen is removed from a metal oxide (e.g., CuO + H₂ → Cu + H₂O), the metal oxide is reduced to the metal. This is the clearest demonstration of reduction among the given options.
Important Facts:
• Oxidation = addition of oxygen or removal of hydrogen/electrons.
• Reduction = removal of oxygen or addition of hydrogen/electrons.
• Redox reactions involve simultaneous oxidation and reduction.
Correct Answer: Option 2 – Oxygen is removed from a metal oxide during heating
RRB Group D 2025-26 – 18 August 2026 Shift 1 · Oxidation, reduction and redox reactions

Q11. Why do oxidation and reduction reactions always occur together in chemical changes?

  1. (a) Reaction must be balanced
  2. (b) One substance loses what another gains
  3. (c) Products must be neutral
  4. (d) Energy conservation law demands it
Answer & solution

Answer: (b) One substance loses what another gains

Why oxidation and reduction occur together
Oxidation is the loss of electrons and reduction is the gain of electrons. In any redox reaction, the electrons lost by one substance (oxidation) must be exactly gained by another substance (reduction) — electrons cannot simply disappear.
This is why oxidation and reduction always occur simultaneously, as complementary halves of the same electron-transfer process — what one substance loses, another gains.
Correct Answer: One substance loses what another gains
RRB Group D 2025-26 – 17 August 2026 Shift 1 · Displacement reactions

Q12. Which visible change confirms displacement reaction when iron reacts with copper sulphate solution?

  1. (a) Solution temperature drops suddenly
  2. (b) Colourless gas evolves rapidly
  3. (c) White precipitate forms instantly
  4. (d) Reddish-brown copper deposits appear
Answer & solution

Answer: (d) Reddish-brown copper deposits appear

Displacement Reaction: Iron and Copper Sulphate
Reaction: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Iron, being more reactive than copper, displaces copper from copper sulphate solution.
The visible confirmation of this displacement reaction is the appearance ofreddish-brown copper metal depositson the iron surface, while the blue colour of the copper sulphate solution fades (turns pale green due to FeSO₄ formation).
Correct Answer: Reddish-brown copper deposits appear
RRB Group D 2025-26 – 17 August 2026 Shift 2 · Combination reactions

Q13. What type of chemical reaction is represented by CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat?

  1. (a) Displacement reaction
  2. (b) Decomposition reaction
  3. (c) Double displacement reaction
  4. (d) Combination reaction
Answer & solution

Answer: (d) Combination reaction

Type of Reaction: CaO + H₂O → Ca(OH)₂
In this reaction, two reactants (calcium oxide and water) combine to form a single product (calcium hydroxide), releasing heat. A reaction in which two or more substances combine to form a single product is acombination reaction(this one is also exothermic).
• It is not a decomposition reaction (single substance breaking into multiple products) — here two reactants combine into one product.
• It is not a displacement or double displacement reaction, as there is no exchange of ions between two compounds.
Correct Answer: Combination reaction
RRB Group D 2025-26 – 14 August 2026 Shift 3 · Double displacement and precipitation reactions

Q14. What is the main characteristic of a double displacement reaction?

  1. (a) Combination of two elements
  2. (b) Gain of oxygen by a substance
  3. (c) Exchange of ions between reactants
  4. (d) Removal of hydrogen from acid
Answer & solution

Answer: (c) Exchange of ions between reactants

Answer: Option 3 – Exchange of ions between reactants.
• Adouble displacementreaction involves theexchange of ionsbetween two reactants.
RRB Group D 2025-26 – 13 August 2026 Shift 1 · Writing and balancing chemical equations

Q15. A student tries to balance the equation Fe + H₂O → Fe₃O₄ + H₂ and writes the following step:Fe + H₂O₄ → Fe₃O₄ + H₂Which statement best explains why this step is INCORRECT?

  1. (a) It balances oxygen atoms directly
  2. (b) It increases oxygen atoms correctly
  3. (c) It changes the composition of water
  4. (d) It maintains mass conservation law
Answer & solution

Answer: (c) It changes the composition of water

Answer: Option 3 – It changes the composition of water.
• Writing H₂O₄ wrongly alters water's formula; you must balance by coefficients, not by changing formulae.
RRB Group D 2025-26 – 13 August 2026 Shift 2 · Oxidation, reduction and redox reactions

Q16. Which change correctly represents oxidation in a chemical reaction?

  1. (a) Removal of oxygen from a compound
  2. (b) Addition of oxygen to a substance
  3. (c) Exchange of ions between compounds
  4. (d) Formation of salt and water
Answer & solution

Answer: (b) Addition of oxygen to a substance

Answer: Option 2 – Addition of oxygen to a substance.
•Oxidation= addition of oxygen (or loss of hydrogen/electrons).
RRB Group D 2025-26 – 13 August 2026 Shift 3 · Double displacement and precipitation reactions

Q17. A chemist mixes two colorless aqueous solutions X and Y. A white precipitate Z forms instantly. Later, analysis shows Z is insoluble calcium carbonate.Which pair of solutions was most likely used?

  1. (a) HCl and NaOH
  2. (b) Na₂CO₃ and CaCl₂
  3. (c) ZnSO₄ and FeCl₂
  4. (d) NaCl and KNO₃
Answer & solution

Answer: (b) Na₂CO₃ and CaCl₂

Answer: Option 2 – Na₂CO₃ and CaCl₂.
• Na₂CO₃ + CaCl₂ → CaCO₃↓ (white precipitate) + 2NaCl.
RRB Group D 2025-26 – 12 August 2026 Shift 3 · Corrosion and rancidity

Q18. What chemical process mainly causes unpleasant smell and taste in oily food items?

  1. (a) Fermentation of sugars
  2. (b) Oxidation of fats and oils
  3. (c) Action of enzymes
  4. (d) Evaporation of water
Answer & solution

Answer: (b) Oxidation of fats and oils

Answer: Option 2 – Oxidation of fats and oils.
•Rancidity(oxidation of fats/oils) causes the bad smell and taste in oily food.
RRB Group D 2025-26 – 11 August 2026 Shift 1 · Decomposition reactions (thermal, electrolytic, photolytic)

Q19. Refer to the two statements given below and choose the CORRECT option.Statement A: A decomposition reaction occurs when a single compound breaks down into two or more simpler substances.Statement B: Decomposition reactions always require the presence of oxygen.

  1. (a) Statement A is incorrect but B is correct.
  2. (b) Both statements A and B are incorrect.
  3. (c) Statement A is correct but B is incorrect.
  4. (d) Both statements A and B are correct.
Answer & solution

Answer: (c) Statement A is correct but B is incorrect.

Answer: Option 3 – Statement A is correct but B is incorrect.
• A is correct: decomposition = one compound → two or more simpler substances.
• B is wrong: decomposition may use heat, light or electricity –oxygen is not always required.
RRB Group D 2025-26 – 11 August 2026 Shift 3 · Exothermic and endothermic reactions

Q20. Which of the following statements correctly explains the relation between a combination reaction and energy?

  1. (a) Combination reactions neither release nor absorb energy
  2. (b) All combination reactions release heat
  3. (c) All combination reactions must absorb heat
  4. (d) Many combination reactions are exothermic, though some can be endothermic
Answer & solution

Answer: (d) Many combination reactions are exothermic, though some can be endothermic

Answer: Option 4 – Many combination reactions are exothermic, though some can be endothermic.
• Most combination reactions release heat (exothermic), but a few absorb heat, so option 4 is the correct general statement.
RRB Group D 2025-26 – 10 August 2026 Shift 1 · Combination reactions

Q21. Which observation confirms that a reaction is a combination reaction?

  1. (a) Gas is evolved during reaction
  2. (b) Temperature decreases suddenly
  3. (c) Only one new substance is produced
  4. (d) Colour of reactants changes
Answer & solution

Answer: (c) Only one new substance is produced

Answer: Option 3 — Only one new substance is produced
A combination reaction formsa single productfrom two or more reactants → Option 3.
RRB Group D 2025-26 – 10 August 2026 Shift 2 · Writing and balancing chemical equations

Q22. During balancing of the following equation, a student first balances hydrogen instead of oxygen.Fe + H₂O → Fe₃O₄ + H₂What is the main difficulty this approach may cause?

  1. (a) Law of conservation becomes invalid
  2. (b) Oxygen atoms remain highly unequal
  3. (c) Chemical formulas must be modified
  4. (d) Coefficients become impossible values
Answer & solution

Answer: (b) Oxygen atoms remain highly unequal

Answer: Option 2 – Oxygen atoms remain highly unequal.
• In Fe + H₂O → Fe₃O₄ + H₂, oxygen appears in H₂O and Fe₃O₄. If a student balances hydrogen first, the oxygen count stays badly unbalanced and repeated re-adjustment is needed.
• Balancing the element that occurs in the fewest compounds/most complex form (here O) first is the standard, easier approach.
• The law of conservation is never invalid; formulas are never changed; coefficients stay whole numbers – so 1, 3, 4 are wrong.
RRB Group D 2025-26 – 10 August 2026 Shift 3 · Double displacement and precipitation reactions

Q23. If you mix aqueous solutions of sodium sulphate and barium chloride, what type of chemical reaction occurs and what is the evidence for it?

  1. (a) Single displacement; color of substance changes
  2. (b) Decomposition; heat release
  3. (c) Double displacement; formation of a precipitate
  4. (d) Redox reaction; gas evolution occurs
Answer & solution

Answer: (c) Double displacement; formation of a precipitate

Answer: Option 3 – Double displacement; formation of a precipitate.
• Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl. Ions exchange partners (double displacement); the whiteprecipitateof BaSO₄ is the evidence.
RRB Group D 2025-26 – 10 August 2026 Shift 3 · Writing and balancing chemical equations

Q24. Which of the following reactions violates the Law of Conservation of Mass?

  1. (a) Ca + Cl₂ → CaCl₂
  2. (b) H₂ + Cl₂ → 2HCl
  3. (c) 2H₂ + O₂ → 2H₂O
  4. (d) 2Mg + O₂ → MgO
Answer & solution

Answer: (d) 2Mg + O₂ → MgO

Answer: Option 4 – 2Mg + O₂ → MgO (unbalanced).
• This equation is not balanced (2 Mg on left, 1 on right; 2 O on left, 1 on right), so as written it violates conservation of mass. The balanced form is 2Mg + O₂ → 2MgO.
RRB Group D 2025-26 – 9 August 2026 Shift 1 · Decomposition reactions (thermal, electrolytic, photolytic)

Q25. What fundamental change distinguishes a decomposition reaction from other types of chemical reactions?

  1. (a) One compound breaks into simpler substances
  2. (b) Ions exchange places in solution
  3. (c) One element replaces another element
  4. (d) Two substances combine to form one
Answer & solution

Answer: (a) One compound breaks into simpler substances

Answer: Option 1 — One compound breaks into simpler substances
Adecompositionreaction: one compound splits into two or more simpler substances → Option 1.
RRB Group D 2025-26 – 9 August 2026 Shift 2 · Corrosion and rancidity

Q26. A packet of potato chips is filled with nitrogen gas before sealing. What is the main reason for using nitrogen in the packet?

  1. (a) Nitrogen reacts with oil to stabilize it
  2. (b) Nitrogen increases the crispness of chips
  3. (c) Nitrogen prevents oxidation of oils and fats
  4. (d) Nitrogen removes moisture from chips
Answer & solution

Answer: (c) Nitrogen prevents oxidation of oils and fats

Answer: Option 3 — Nitrogen prevents oxidation of oils and fats
Inert nitrogen preventsoxidation (rancidity)of the chips' oil → Option 3.
RRB Group D 2025-26 – 6 August 2026 Shift 1 · Displacement reactions

Q27. A student dips an iron nail in a blue solution of copper sulphate. After 30 minutes, the blue colour fades and a reddish-brown deposit forms on the nail. The reddish-brown deposit is due to _______.

  1. (a) Fe converting to copper oxide
  2. (b) copper sulphate converting to iron oxide (rust)
  3. (c) copper sulphate converting to metallic copper
  4. (d) copper sulphate converting to Cu⁺
Answer & solution

Answer: (c) copper sulphate converting to metallic copper

Answer: Option 3 — copper sulphate converting to metallic copper
Fe displaces Cu: Fe + CuSO₄ → FeSO₄ + Cu. The reddish-brown deposit ismetallic copper→ Option 3.
RRB Group D 2025-26 – 6 August 2026 Shift 1 · Double displacement and precipitation reactions

Q28. Consider the reaction:Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)Which statement best justifies classifying this reaction as a double displacement reaction?

  1. (a) Exchange of ions produces two new compounds
  2. (b) Reactants combine forming only one product
  3. (c) Heat causes compound to break into products
  4. (d) One metal displaces another from compound
Answer & solution

Answer: (a) Exchange of ions produces two new compounds

Answer: Option 1 — Exchange of ions produces two new compounds
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl: ions areexchangedto form two new compounds — a double displacement reaction → Option 1.
RRB Group D 2025-26 – 5 August 2026 Shift 1 · Oxidation, reduction and redox reactions

Q29. Consider the reaction: CuO(s) + H₂(g) → Cu(s) + H₂O(l)Which option correctly identifies the oxidation–reduction changes occurring in this reaction?

  1. (a) Hydrogen oxidised while CuO reduced
  2. (b) Copper oxidised while H₂ reduced
  3. (c) Copper and hydrogen both oxidised
  4. (d) Hydrogen reduced while CuO oxidised
Answer & solution

Answer: (a) Hydrogen oxidised while CuO reduced

Answer: Option 1 — Hydrogen oxidised while CuO reduced
CuO loses oxygen (reduced to Cu); H₂ gains oxygen (oxidised to H₂O) → Option 1.
RRB Group D 2025-26 – 5 August 2026 Shift 2 · Oxidation, reduction and redox reactions

Q30. Consider the reaction:MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂Based on the oxygen–hydrogen definition of redox reactions, which substance undergoes reduction in this reaction?

  1. (a) Water forms after oxygen gain
  2. (b) Manganese dioxide loses oxygen
  3. (c) Chlorine gas forms by oxidation
  4. (d) Hydrogen chloride loses hydrogen
Answer & solution

Answer: (b) Manganese dioxide loses oxygen

Answer: Option 2 — Manganese dioxide loses oxygen
In MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂,MnO₂ loses oxygen(Mn goes +4 → +2), so it isreduced→ Option 2.
RRB Group D 2025-26 – 5 August 2026 Shift 3 · Decomposition reactions (thermal, electrolytic, photolytic)

Q31. Which energy source is essential to initiate most decomposition reactions successfully?

  1. (a) Mechanical stirring force
  2. (b) Atmospheric pressure alone
  3. (c) Presence of catalyst only
  4. (d) Heat, light or electrical energy
Answer & solution

Answer: (d) Heat, light or electrical energy

Answer: Option 4 — Heat, light or electrical energy
Decomposition reactions are driven byheat, light or electricity(thermal/photo/electrolytic) → Option 4.
RRB Group D 2025-26 – 4 August 2026 Shift 1 · Double displacement and precipitation reactions

Q32. A reaction occurs when two aqueous salt solutions are mixed, and an insoluble solid forms while the remaining ions stay dissolved in solution. Which type of reaction does this represent?

  1. (a) Decomposition reaction requiring energy input
  2. (b) Combination reaction forming single product
  3. (c) Double displacement precipitation reaction
  4. (d) Displacement reaction involving metal exchange
Answer & solution

Answer: (c) Double displacement precipitation reaction

Answer: Option 3 — Double displacement precipitation reaction
Two salt solutions exchange ions and form aninsoluble precipitate→ adouble displacement (precipitation)reaction → Option 3.
RRB Group D 2025-26 – 4 August 2026 Shift 2 · Displacement reactions

Q33. When a zinc strip is dipped into a solution of iron(II) sulphate, what will happen?

  1. (a) Iron deposits on zinc, and hydrogen gas is evolved.
  2. (b) Zinc gets coated with iron oxide.
  3. (c) No reaction occurs
  4. (d) Zinc displaces iron from the solution.
Answer & solution

Answer: (d) Zinc displaces iron from the solution.

Answer: Option 4 — Zinc displaces iron from the solution
Zinc is more reactive than iron, soZn displaces Fe: Zn + FeSO₄ → ZnSO₄ + Fe → Option 4.
RRB Group D 2025-26 – 4 August 2026 Shift 3 · Corrosion and rancidity

Q34. Which storage condition slows rancidity most effectively in edible oils and fats?

  1. (a) Mixing food with water
  2. (b) Keeping under sunlight
  3. (c) Storing in airtight containers
  4. (d) Repeated heating of food
Answer & solution

Answer: (c) Storing in airtight containers

Answer: Option 3 — Storing in airtight containers
Airtight storage keeps out oxygen, slowingrancidity (oxidation)of oils/fats → Option 3.
RRB Group D 2025-26 – 3 August 2026 Shift 1 · Decomposition reactions (thermal, electrolytic, photolytic)

Q35. Consider the following reactions:(I) CaCO₃(s) —Heat→ CaO(s) + CO₂(g)(II) 2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g)(III) 2H₂O(l) —Electric current→ 2H₂(g) + O₂(g)(IV) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)Which of the above reactions correctly represent decomposition reactions?

  1. (a) Only I and II reactions
  2. (b) Only III and IV reactions
  3. (c) Only II and III reactions
  4. (d) Only I, II and III reactions
Answer & solution

Answer: (d) Only I, II and III reactions

Answer: Option 4 — Only I, II and III reactions
Concept:A *decomposition* reaction is one where a single compound breaks into two or more simpler substances (AB → A + B), usually using heat (thermal), light (photo) or electricity (electrolytic).
Check each:
• (I) CaCO₃ → CaO + CO₂ (heat) — decomposition ✓ (thermal)
• (II) 2AgCl → 2Ag + Cl₂ (sunlight) — decomposition ✓ (photo)
• (III) 2H₂O → 2H₂ + O₂ (electric current) — decomposition ✓ (electrolytic)
• (IV) Fe + CuSO₄ → FeSO₄ + Cu — this isdisplacement(Fe displaces Cu), NOT decomposition ✗
So I, II and III are decomposition reactions →Option 4.
RRB Group D 2025-26 – 3 August 2026 Shift 3 · Oxidation, reduction and redox reactions

Q36. A student observes the following reaction during heating of copper powder in air.2Cu + O₂ → 2CuOLater, hydrogen gas is passed over the black copper oxide and reddish-brown copper is obtained.CuO + H₂ → Cu + H₂OWhich substance is o̲x̲i̲d̲i̲z̲e̲d̲ ̲i̲n̲ ̲f̲i̲r̲s̲t̲ ̲r̲e̲a̲c̲t̲i̲o̲n̲ ̲a̲n̲d̲ ̲r̲e̲d̲u̲c̲e̲d̲ ̲i̲n̲ ̲s̲e̲c̲o̲n̲d̲ ̲r̲e̲a̲c̲t̲i̲o̲n̲?

  1. (a) Copper in first reaction and Copper oxide in second reaction
  2. (b) Hydrogen in first reaction and Copper oxide in second reaction
  3. (c) Copper oxide in first reaction and Copper in second reaction
  4. (d) Oxygen in first reaction and Copper oxide in second reaction
Answer & solution

Answer: (a) Copper in first reaction and Copper oxide in second reaction

Answer: Option 1 — Copper in the first reaction and Copper oxide in the second
Reaction 1: 2Cu + O₂ → 2CuO — Cu gains oxygen ⟹Cu is oxidised.
Reaction 2: CuO + H₂ → Cu + H₂O — CuO loses oxygen ⟹CuO is reduced.
→ Option 1.
RRB NTPC UG CBT-1 2025-26 – 19 June 2026 Shift 2 · Double displacement and precipitation reactions

Q37. When Barium Chloride (BaCl₂) solution is mixed with Sodium Sulfate (Na₂SO₄) solution, a white product Barium Sulfate (BaSO₄) is formed. This product is known as a ______.

  1. (a) Isotope
  2. (b) Emulsion
  3. (c) Isomer
  4. (d) Precipitate
Answer & solution

Answer: (d) Precipitate

Reaction: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
Precipitate:Insoluble solid formed in solution
BaSO₄ isinsoluble in waterand settles as white solid
Other options:
Isotope → Same element, different mass
Emulsion → Liquid-liquid mixture
Isomer → Same formula, different structure
Answer: Option 4 → Precipitate
RRB NTPC UG CBT-1 2025-26 – 18 June 2026 Shift 1 · Oxidation, reduction and redox reactions

Q38. Oxidation in a redox reaction involves which of the following?

  1. (a) Electrons lost
  2. (b) Protons gained
  3. (c) Protons lost
  4. (d) Electrons gained
Answer & solution

Answer: (a) Electrons lost

Redox Reaction - Oxidation Process
Oxidation is defined as theloss of electrons.
Mnemonic:OIL RIG
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
Example: Fe → Fe²⁺ + 2e⁻ (oxidation)
Answer: Option 1 → Electrons lost
RRB NTPC UG CBT-1 2025-26 – 17 June 2026 Shift 3 · Displacement reactions

Q39. During an experiment, iron nails are placed in copper sulphate solution, and a reddish-brown deposit forms on the nails. What does this observation demonstrate about the type of chemical reaction and the substances involved?

  1. (a) A decomposition reaction occurs, breaking copper sulphate into SO₂ and copper
  2. (b) A double displacement reaction forms iron sulphate and copper sulphate
  3. (c) A displacement reaction occurs, forming copper metal on the iron nails
  4. (d) A combination reaction occurs, creating iron sulphate crystals
Answer & solution

Answer: (c) A displacement reaction occurs, forming copper metal on the iron nails

Observation: Iron nails in CuSO₄ solution → reddish-brown deposit forms
Reaction Type: Displacement (Single Replacement)
Iron is more reactive than copper in reactivity series
Fe displaces Cu from CuSO₄ solution
Chemical Equation:
Fe + CuSO₄ → FeSO₄ + Cu↓
Reddish-brown deposit = Copper metal (Cu)
Solution turns light green = Iron sulphate (FeSO₄) formed
Answer: Option 3 → A displacement reaction occurs, forming copper metal on the iron nails
RRB NTPC UG CBT-1 2025-26 – 15 June 2026 Shift 1 · Decomposition reactions (thermal, electrolytic, photolytic)

Q40. What is/are the element(s) or compound(s) that can be used in black and white photography?

  1. (a) Only AgCl
  2. (b) Only AgBr
  3. (c) Only Ag
  4. (d) AgCl and AgBr
Answer & solution

Answer: (d) AgCl and AgBr

Black and White Photography - Light-Sensitive Compounds
Photographic film usessilver halidesthat are sensitive to light.
Silver Chloride (AgCl)- decomposes when exposed to light
Silver Bromide (AgBr)- most commonly used in photographic films
Both compounds turn dark when exposed to light due to formation of metallic silver.
Pure silver (Ag) is the product, not the light-sensitive material used.
Answer: Option 4 → AgCl and AgBr
RRB NTPC UG CBT-1 2025-26 – 15 June 2026 Shift 2 · Displacement reactions

Q41. In a laboratory, copper sulphate solution reacts with iron filings, causing a colour change and the deposition of a new metal. Which statement correctly explains the reason for this reaction?

  1. (a) Iron and copper sulphate have similar reactivity
  2. (b) Iron and copper belong to the same group
  3. (c) Iron is more reactive than copper
  4. (d) Copper is more reactive than iron
Answer & solution

Answer: (c) Iron is more reactive than copper

Reaction: CuSO₄ + Fe → FeSO₄ + Cu
Observation: Blue CuSO₄ solution turns green (FeSO₄)
Reddish-brown copper deposits on iron filings
Reason: Displacement Reaction
More reactive metal displaces less reactive metal from its salt
Reactivity Series: Fe > Cu
Iron displaces copper from copper sulphate
Answer: Option 3 → Iron is more reactive than copper
RRB NTPC UG CBT-1 2025-26 – 14 June 2026 Shift 1 · Displacement reactions

Q42. Why does iron replace copper when an iron nail is dipped in copper sulphate solution?

  1. (a) Iron is less reactive
  2. (b) Both are equally reactive
  3. (c) Copper is insoluble in water
  4. (d) Iron is more reactive
Answer & solution

Answer: (d) Iron is more reactive

Reactivity Series Concept
When iron nail is dipped in CuSO₄ solution:
Reaction: Fe + CuSO₄ → FeSO₄ + Cu
Iron displaces copper from copper sulphate.
Blue color of CuSO₄ fades, reddish-brown Cu deposits on nail.
Reason:Iron is more reactive than copper in reactivity series.
More reactive metal displaces less reactive metal from its salt solution.
Answer: Option 4 → Iron is more reactive
RRB NTPC UG CBT-1 2025-26 – 14 June 2026 Shift 1 · Double displacement and precipitation reactions

Q43. A student mixes aqueous solutions of barium chloride and sodium sulphate. A white curd-like substance forms instantly. What type of chemical reaction is primarily responsible for this observation?

  1. (a) Decomposition of sodium sulphate by barium chloride catalyst
  2. (b) Evolution of SO₂ gas during a displacement reaction
  3. (c) Precipitation reaction forming insoluble barium sulphate
  4. (d) Redox reaction resulting in electron transfer
Answer & solution

Answer: (c) Precipitation reaction forming insoluble barium sulphate

Reaction: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
When barium chloride and sodium sulphate solutions mix, barium ions (Ba²⁺) react with sulphate ions (SO₄²⁻).
BaSO₄ is highly insoluble in waterand precipitates as a white curd-like substance.
This is adouble displacement precipitation reactionwhere two soluble salts exchange ions to form an insoluble product.
No decomposition, gas evolution, or electron transfer occurs.
Answer: Option 3 → Precipitation reaction forming insoluble barium sulphate
RRB NTPC UG CBT-1 2025-26 – 9 May 2026 Shift 2 · Double displacement and precipitation reactions

Q44. In the reaction Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(*) + 2NaCl(aq), what will be the actual physical state that can replace (*)?

  1. (a) (g)
  2. (b) (aq)
  3. (c) (s)
  4. (d) (l)
Answer & solution

Answer: (c) (s)

Reaction: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(*) + 2NaCl(aq)
Double Displacement Reaction
BaSO₄ (Barium sulfate) isinsoluble in water
Forms white precipitate when Ba²⁺ and SO₄²⁻ ions combine
Physical state:Solid (s)
Answer: Option 3 → (s)
RRB NTPC UG CBT-1 2025-26 – 7 May 2026 Shift 1 · Decomposition reactions (thermal, electrolytic, photolytic)

Q45. During the electrolysis of water, hydrogen and oxygen are released in which ratio?

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

Answer: (b) 2:1

Electrolysis of Water: 2H₂O → 2H₂ + O₂
From the balanced equation:
2 molecules of H₂O produce 2 molecules of H₂ and 1 molecule of O₂
Volume ratio =H₂ : O₂ = 2 : 1
At cathode: Hydrogen gas (H₂) is released
At anode: Oxygen gas (O₂) is released
Answer: Option 2 → 2:1
RRB NTPC GL CBT-1 2025-26 – 27 March 2026 Shift 3 · Decomposition reactions (thermal, electrolytic, photolytic)

Q46. Which type of chemical reaction involves the breakdown of a compound into two or more simpler substances?

  1. (a) Displacement reaction
  2. (b) Neutralization reaction
  3. (c) Decomposition reaction
  4. (d) Combination reaction
Answer & solution

Answer: (c) Decomposition reaction

Chemical Reaction Types:
Decomposition Reaction:A single compound breaks down into two or more simpler substances.
General form: AB → A + B
Example: 2H₂O → 2H₂ + O₂ (water breaks into hydrogen and oxygen)
Other reactions:
• Combination: A + B → AB (opposite of decomposition)
• Displacement: A + BC → AC + B (one element replaces another)
• Neutralization: Acid + Base → Salt + Water
Answer: Option 3 → Decomposition reaction
RRB NTPC GL CBT-1 2025-26 – 19 March 2026 Shift 3 · Corrosion and rancidity

Q47. What leads to rancidity in food products?

  1. (a) Oxidation
  2. (b) Evaporation
  3. (c) Fermentation
  4. (d) Reduction
Answer & solution

Answer: (a) Oxidation

Rancidity in Food Products
Rancidity is the spoilage of fats and oils in food.
Cause:Oxidation of unsaturated fats by atmospheric oxygen.
This produces unpleasant smell and taste.
Prevention:Antioxidants, airtight packaging, refrigeration.
Answer: Option 1 → Oxidation
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 1 · Writing and balancing chemical equations

Q48. In the chemical equation xPb(NO₃)₂(s) → yPbO(s) + zNO₂(g) + wO₂(g), what are the values of x, y, z and w?

  1. (a) x = 2 ; y = 4 ; z = 2 ; w = 1
  2. (b) x = 2 ; y = 1 ; z = 4 ; w = 2
  3. (c) x = 2 ; y = 2 ; z = 4 ; w = 1
  4. (d) x = 1 ; y = 2 ; z = 3 ; w = 2
Answer & solution

Answer: (c) x = 2 ; y = 2 ; z = 4 ; w = 1

The reaction is the thermal decomposition of lead nitrate: Pb(NO₃)₂ (s) → PbO (s) + NO₂ (g) + O₂ (g).
Balance Pb atoms: left 1 Pb, right 1 Pb → x = y.
Balance N atoms: Left has 2 N per Pb(NO₃)₂, so 2x N atoms.
Right side NO₂ has z N atoms → z = 2x.
Balance O atoms:
Left: Each Pb(NO₃)₂ has 6 O atoms × x = 6x O atoms.
Right: PbO has y O atoms = y O atoms.
NO₂ has 2 O atoms each × z = 2z O atoms.
O₂ has 2 O atoms each × w = 2w O atoms.
So, total O on right = y + 2z + 2w = 6x.
Use x = y and z = 2x:
y + 2(2x) + 2w = 6x
x + 4x + 2w = 6x
5x + 2w = 6x
2w = 6x − 5x = x
w = x/2.
Since w must be integer, choose x=2 for smallest integer values.
Then:
x=2, y=2, z=4, w=1.
Hence balanced equation: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 2 · Exothermic and endothermic reactions

Q49. The burning of carbon to form carbon dioxide is an example of a/an:

  1. (a) Endothermic reaction
  2. (b) Photochemical reaction
  3. (c) Reversible reaction
  4. (d) Exothermic reaction
Answer & solution

Answer: (d) Exothermic reaction

The burning of carbon forms carbon dioxide by reacting carbon with oxygen.
This process releases heat to the surroundings.
A reaction that releases heat is called an exothermic reaction.
Therefore, burning carbon to form carbon dioxide is an exothermic reaction.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 1 · Oxidation, reduction and redox reactions

Q50. Which of the following best defines reduction in a redox reaction?

  1. (a) Gain of both oxygen and electrons
  2. (b) Gain of oxygen or loss of hydrogen
  3. (c) Gain of hydrogen or loss of oxygen
  4. (d) Loss of electrons only
Answer & solution

Answer: (c) Gain of hydrogen or loss of oxygen

Reduction in a redox reaction refers to the process where a substance gains hydrogen or loses oxygen.
This is because gaining hydrogen typically means gaining electrons, and losing oxygen also indicates gaining electrons.
Hence, option 3 correctly defines reduction.
Other options are incorrect as reduction is not just loss of electrons nor gain of oxygen.
Therefore, the best definition is 'Gain of hydrogen or loss of oxygen'.
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 2 · Double displacement and precipitation reactions

Q51. 1. A precipitation reaction occurs when two aqueous solutions react to form an insoluble solid.
2. The solid formed is called a precipitate.
3. All precipitation reactions involve acids and bases only.
Which of the above statements are correct?

  1. (a) 2 and 3 only
  2. (b) 1, 2, and 3
  3. (c) 1 and 3 only
  4. (d) 1 and 2 only
Answer & solution

Answer: (d) 1 and 2 only

Statement 1 is correct: Precipitation reaction occurs when two aqueous solutions form an insoluble solid.
Statement 2 is correct: The solid formed in such reaction is called a precipitate.
Statement 3 is incorrect: Not all precipitation reactions involve acids and bases; they generally involve ionic compounds forming insoluble salts.
Hence, only statements 1 and 2 are correct.
Final answer: Option 4
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 1 · Double displacement and precipitation reactions

Q52. Which of the following is an example of a double displacement reaction?

  1. (a) 2Mg + O₂ → 2MgO
  2. (b) H₂ + Cl₂ → 2HCl
  3. (c) Na2O + H2O→ 2 NaOH
  4. (d) AgNO₃ + NaCl → AgCl + NaNO₃
Answer & solution

Answer: (d) AgNO₃ + NaCl → AgCl + NaNO₃

Option 4 is correct.
A double displacement reaction involves the exchange of ions between two compounds to form new compounds.
In option 4, AgNO₃ (silver nitrate) and NaCl (sodium chloride) exchange their ions to form AgCl (silver chloride) and NaNO₃ (sodium nitrate).
This is a classic example of a double displacement reaction.
Options 1 and 2 are synthesis reactions (combination).
Option 3 is a combination reaction forming a base, not a double displacement.
Hence, the correct answer is Option 4.
Answer complete.
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 1 · Double displacement and precipitation reactions

Q53. Which of the following is an example of a double displacement reaction?

  1. (a) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
  2. (b) CaCO₃ → CaO + CO₂
  3. (c) Fe + CuSO₄ → FeSO₄ + Cu
  4. (d) 2H₂ + O₂ → 2H₂O
Answer & solution

Answer: (a) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Option 1 shows a double displacement reaction.
In this reaction, Na₂SO₄ and BaCl₂ exchange their ions to form BaSO₄ and NaCl.
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl is the correct example.
Option 2 is decomposition, Option 3 is a single displacement reaction, and Option 4 is a combination reaction.
Hence, the answer is Option 1.
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 3 · Double displacement and precipitation reactions

Q54. A student adds lead (II) nitrate (Pb(NO₃)₂) to an unknown solution. A yellow precipitate forms. What could the unknown solution be?

  1. (a) Copper sulphate (CuSO4)
  2. (b) Sodium chloride (NaCl)
  3. (c) Potassium iodide (KI)
  4. (d) Barium sulphate (BaSO4)
Answer & solution

Answer: (c) Potassium iodide (KI)

The yellow precipitate formed when lead (II) nitrate is added indicates the formation of lead iodide (PbI2).
Lead iodide is a bright yellow solid formed by the reaction of Pb(NO3)2 with potassium iodide (KI).
Thus, the unknown solution must contain iodide ions, i.e., it is potassium iodide (KI).
Final answer: Option 3
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 2 · Oxidation, reduction and redox reactions

Q55. In a redox reaction, what is the role of a 'reducing agent'?

  1. (a) It is a substance that only participates in decomposition reactions.
  2. (b) It is a catalyst that speeds up the reaction.
  3. (c) It is a substance that causes reduction and itself gets oxidized.
  4. (d) It is an agent that helps other substances to be oxidized.
Answer & solution

Answer: (c) It is a substance that causes reduction and itself gets oxidized.

The reducing agent is the substance that causes reduction in other substances.
It donates electrons to another species and itself loses electrons.
Losing electrons means the reducing agent itself gets oxidized.
Hence, a reducing agent is a substance that causes reduction and itself gets oxidized.
Correct option is 3.
Answer complete.
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 2 · Corrosion and rancidity

Q56. Why is nitrogen gas used to prevent oxidation in bags containing foods such as chips?

  1. (a) Nitrogen gas inhibits oxidation by replacing oxygen.
  2. (b) Nitrogen gas enhances the taste of the food.
  3. (c) Nitrogen gas accelerates oxidation.
  4. (d) Nitrogen gas makes chips crispier.
Answer & solution

Answer: (a) Nitrogen gas inhibits oxidation by replacing oxygen.

Nitrogen is an inert gas and does not react easily with other substances.
Oxidation occurs when oxygen reacts with the food, causing spoilage and rancidity.
By replacing oxygen with nitrogen, oxidation is prevented or slowed down.
This keeps the food, like chips, fresh and extends shelf life.
Hence, nitrogen gas inhibits oxidation by replacing oxygen inside the packaging.
Final answer: Option 1
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 3 · Corrosion and rancidity

Q57. A bright metal turns dull over time due to oxidation. What is this process called?

  1. (a) Corrosion
  2. (b) Filtration
  3. (c) Evaporation
  4. (d) Sublimation
Answer & solution

Answer: (a) Corrosion

Option 1 is correct.
The process in which a bright metal turns dull due to oxidation is called corrosion.
Corrosion is the gradual destruction of metals by chemical reactions with their environment, mainly oxygen.
Hence, the correct term for this process is corrosion.
Answer complete.
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 2 · Corrosion and rancidity

Q58. Which method helps prevent rancidity?

  1. (a) Exposing food to sunlight
  2. (b) Storing food in airtight containers
  3. (c) Heating food continuously
  4. (d) Adding excess water
Answer & solution

Answer: (b) Storing food in airtight containers

Rancidity is the process where fats and oils decompose, causing unpleasant smell and taste.
To prevent rancidity, oxygen must be restricted as it reacts with fats to cause spoilage.
Storing food in airtight containers limits exposure to oxygen and moisture, thus preventing rancidity efficiently.
Other methods like exposing food to sunlight or continuous heating can accelerate spoilage, not prevent it.
Adding excess water does not prevent rancidity and may promote microbial growth.
Therefore, the best method is storing food in airtight containers.
Final answer: Option 2
RRB NTPC UG CBT-1 2024-25 – 2 September 2025 Shift 1 · Rate of reaction and catalysts

Q59. Which of the following metals is used as a catalyst in the Haber process for ammonia synthesis?

  1. (a) Platinum
  2. (b) Nickel
  3. (c) Copper
  4. (d) Iron
Answer & solution

Answer: (d) Iron

RRB NTPC UG CBT-1 2024-25 – 1 September 2025 Shift 1 · Oxidation, reduction and redox reactions

Q60. Which type of reaction is involved in the rusting of iron?

  1. (a) Displacement
  2. (b) Redox
  3. (c) Combination
  4. (d) Decomposition
Answer & solution

Answer: (b) Redox

RRB NTPC UG CBT-1 2024-25 – 1 September 2025 Shift 3 · Writing and balancing chemical equations

Q61. In a chemical reaction, the substances that are initially present before the reaction starts are called _____.

  1. (a) reactants
  2. (b) products
  3. (c) intermediates
  4. (d) catalysts
Answer & solution

Answer: (a) reactants

RRB NTPC UG CBT-1 2024-25 – 22 August 2025 Shift 3 · Corrosion and rancidity

Q62. Which chemical process results in the formation of plaque on copper, the rusting of iron and the blackening of silver?

  1. (a) Encapsulation
  2. (b) Flocculation
  3. (c) Corrosion
  4. (d) Extrusion
Answer & solution

Answer: (c) Corrosion

RRB NTPC UG CBT-1 2024-25 – 18 August 2025 Shift 2 · Rate of reaction and catalysts

Q63. What is the term used for a substance that reduces the rate of a chemical reaction?

  1. (a) Catalyst
  2. (b) Reactant
  3. (c) Product
  4. (d) Inhibitor
Answer & solution

Answer: (d) Inhibitor

RRB NTPC UG CBT-1 2024-25 – 13 August 2025 Shift 1 · Writing and balancing chemical equations

Q64. Which of the following chemical reactions does NOT represent a balanced equation?

  1. (a) 2Fe + 3H₂O → Fe₂O₃ + 3H₂
  2. (b) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
  3. (c) Fe + H₂O → FeO + H₂
  4. (d) 4Fe + 3H₂O → Fe₃O₄ + 2H₂
Answer & solution

Answer: (d) 4Fe + 3H₂O → Fe₃O₄ + 2H₂

SSC Selection Post 2025 (Phase XIII) – 2 August 2025 Shift 3 · Exothermic and endothermic reactions

Q65. Which among the following is an example of an exothermic reaction?

  1. (a) Photosynthesis
  2. (b) Melting of ice
  3. (c) Combustion of petrol
  4. (d) Evaporation of water
Answer & solution

Answer: (c) Combustion of petrol

Correct Answer: Option 3 – Combustion of petrol
Exothermic reactions release energy (heat) to the surroundings. Combustion of petrol (burning fuel) is a classic exothermic reaction: C₈H₁₈ + O₂ → CO₂ + H₂O + Heat energy. The energy released is what powers internal combustion engines.
Option 1 (Photosynthesis): An endothermic reaction — plants absorb light energy from the Sun to convert CO₂ and water into glucose.
Option 2 (Melting of ice): An endothermic process — heat energy is absorbed from the surroundings to break the bonds in ice crystal lattice.
Option 4 (Evaporation of water): An endothermic process — liquid water absorbs heat energy to overcome intermolecular forces and convert to water vapour .
RRB NTPC GL CBT-1 2024-25 – 23 June 2025 Shift 1 · Oxidation, reduction and redox reactions

Q66. Which of the following is a disproportionation reaction?

  1. (a) 2Na + Cl₂ → 2NaCl
  2. (b) Cu + HNO₃ → Cu(NO₃)₂ + NO₂ + H₂O
  3. (c) Zn + HCl → ZnCl₂ + H₂
  4. (d) Cl₂ + H₂O → HCl + HOCl
Answer & solution

Answer: (d) Cl₂ + H₂O → HCl + HOCl

RRB NTPC GL CBT-1 2024-25 – 17 June 2025 Shift 3 · Chemical equilibrium basics

Q67. The addition of H⁺ ions to the equilibrium CH₃COOH ⇌ CH₃COO⁻ + H⁺ will:

  1. (a) Lead to no change
  2. (b) Shift the equilibrium right
  3. (c) Shift the equilibrium left
  4. (d) Stop the reaction
Answer & solution

Answer: (c) Shift the equilibrium left

RRB NTPC GL CBT-1 2024-25 – 16 June 2025 Shift 1 · Corrosion and rancidity

Q68. What is the advantage of keeping food in an airtight container?

  1. (a) It enhances the taste of the food.
  2. (b) It promotes anaerobic bacterial growth.
  3. (c) It changes the colour of the food.
  4. (d) It slows down oxidation.
Answer & solution

Answer: (d) It slows down oxidation.

RRB NTPC GL CBT-1 2024-25 – 13 June 2025 Shift 2 · Writing and balancing chemical equations

Q69. In the balanced reaction 2Al + 3Cl₂ → 2AlCl₃, what is the mole ratio of aluminium to aluminium chloride?

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

Answer: (a) 2 : 2

RRB NTPC GL CBT-1 2024-25 – 11 June 2025 Shift 2 · Oxidation, reduction and redox reactions

Q70. In acidic medium, the balanced redox reaction for MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺ is:

  1. (a) MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
  2. (b) MnO₄⁻ + Fe²⁺ → MnO₂ + Fe³⁺
  3. (c) MnO₄⁻ + 3Fe²⁺ + 2H⁺ → MnO₂ + Fe³⁺ + H₂O
  4. (d) MnO₄⁻ + 2Fe²⁺ + H₂O → Mn²⁺ + Fe³⁺
Answer & solution

Answer: (a) MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

RRB NTPC GL CBT-1 2024-25 – 10 June 2025 Shift 3 · Corrosion and rancidity

Q71. Oil and fat containing food items are flushed with nitrogen because it:

  1. (a) Removes oxygen from the packaging
  2. (b) Adds nutritional value to the product
  3. (c) Enhances the flavour of the product
  4. (d) Removes hydrogen from the packaging
Answer & solution

Answer: (a) Removes oxygen from the packaging

RRB NTPC GL CBT-1 2024-25 – 6 June 2025 Shift 3 · Chemical equilibrium basics

Q72. Solubility of CH₃COOH decreases in presence of CH₃COONa due to:

  1. (a) Common ion effect
  2. (b) Ion pairing
  3. (c) Increased temperature
  4. (d) Oxidation
Answer & solution

Answer: (a) Common ion effect

RRB Technician Grade III 2024-25 – 30 December 2024 Shift 1 · Writing and balancing chemical equations

Q73. Identify the balanced chemical equation for the following reaction.
Mg + O₂ → MgO

  1. (a) 2Mg + ½O₂ → 2MgO
  2. (b) Mg + ½O₂ → MgO
  3. (c) Mg + 3/2O₂ → MgO
  4. (d) 2Mg + 3/2O₂ → 3MgO
Answer & solution

Answer: (b) Mg + ½O₂ → MgO

Correct Answer: Option 2 — Mg + ½O₂ → MgO
A chemical equation is balanced when the number of atoms of every element is the same on both sides, as demanded by the law of conservation of mass. The skeletal equation Mg + O₂ → MgO has 2 oxygen atoms on the left but only 1 on the right, so it must be balanced.
Burning magnesium ribbon in air is a combination reaction and also an oxidation: the metal takes up oxygen, burning with a dazzling white light and leaving a white powder of magnesium oxide (MgO). This is why the ribbon is first cleaned with sandpaper — to remove the protective MgO layer already on it.
⚗️ The Reaction
Mg + ½O₂ → MgO
• Mg: 1 = 1 and O: ½ × 2 = 1 = 1 — every atom balances.
• Multiplying throughout by 2 gives the whole-number form 2Mg + O₂ → 2MgO, which is the same equation.
• MgO + H₂O → Mg(OH)₂, a base that turns red litmus blue — proof that metal oxides are basic.
❌ Why the other options are incorrect
✗ 2Mg + ½O₂ → 2MgO — magnesium balances (2 = 2) but oxygen does not: ½ × 2 = 1 atom on the left against 2 on the right.
✗ Mg + 3/2O₂ → MgO — the left side carries 3 oxygen atoms against just 1 on the right; 3/2 comes from copying the coefficient used for oxides like Al₂O₃.
✗ 2Mg + 3/2O₂ → 3MgO — here the oxygen happens to balance (3/2 × 2 = 3 = 3), which makes the option look convincing, but magnesium does not: 2 atoms on the left against 3 on the right. One unbalanced element is enough to reject an equation.
📌 SSC & Railway Exam Facts
• Whole-number form: 2Mg + O₂ → 2MgO — a standard combination (synthesis) reaction.
• Burning magnesium gives a blinding white flame; it is used in flash powder, fireworks and flares.
• Magnesium ribbon is cleaned before burning to remove the layer of magnesium oxide.
• Balancing follows the law of conservation of mass, given by Antoine Lavoisier.
• Magnesium is the metal atom at the centre of chlorophyll.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · Oxidation, reduction and redox reactions

Q74. Identify the substance that gets reduced in the given equation:
ZnO + C → Zn + CO

  1. (a) ZnO
  2. (b) CO
  3. (c) Zn
  4. (d) C
Answer & solution

Answer: (a) ZnO

Correct Answer: Option 1 — ZnO
In a redox reaction, oxidation means the gain of oxygen (or loss of hydrogen / loss of electrons) and reduction means the loss of oxygen (or gain of hydrogen / gain of electrons). The two always happen together in the same reaction.
Here zinc oxide, ZnO, loses its oxygen and becomes zinc metal, Zn — so ZnO is reduced, and it acts as the oxidising agent. At the same time carbon, C, gains oxygen and becomes carbon monoxide, CO — so C is oxidised, and it acts as the reducing agent.
This is exactly the smelting step in the extraction of zinc: the ore is first roasted to ZnO and then reduced by coke in a furnace.
⚗️ The Reaction
ZnO + C → Zn + CO
• ZnO → Zn: oxygen is removed ⇒ reduction.
• C → CO: oxygen is added ⇒ oxidation.
• Oxidation number of Zn falls from +2 to 0, while that of C rises from 0 to +2 — the same conclusion by the electron method.
❌ Why the other options are incorrect
✗ CO — carbon monoxide is a product formed by the oxidation of carbon; a product of oxidation cannot be the substance reduced.
✗ Zn — zinc metal is also a product; it is what is obtained after ZnO has been reduced, not the species undergoing reduction.
✗ C — carbon gains oxygen, so carbon is oxidised; it is the reducing agent, the exact opposite of what is asked.
📌 SSC & Railway Exam Facts
• Oxidation = gain of oxygen / loss of hydrogen / loss of electrons (OIL); reduction = loss of oxygen / gain of hydrogen / gain of electrons (RIG).
• The oxidising agent is itself reduced; the reducing agent is itself oxidised.
• Carbon (coke) is the commonest reducing agent used in metallurgy; ZnO + C → Zn + CO.
• Highly reactive metals (K, Na, Ca, Mg, Al) cannot be reduced by carbon — they are obtained by electrolytic reduction.
• Thermite reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat, used for welding rail joints — here Al is the reducing agent.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Double displacement and precipitation reactions

Q75. Which of the following reactions is an example of double displacement reaction?

  1. (a) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
  2. (b) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
  3. (c) Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)
  4. (d) Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
Answer & solution

Answer: (d) Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Correct Answer: Option 4 — Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
In a double displacement reaction, two compounds exchange their ions (radicals) with each other to form two new compounds. Its general pattern is AB + CD → AD + CB. The tell-tale sign is that both reactants are compounds — no free element takes part.
In option 4, sodium sulphate and barium chloride simply swap partners: Ba²⁺ joins SO₄²⁻ to give barium sulphate, and Na⁺ joins Cl⁻ to give sodium chloride. Because BaSO₄ is insoluble, it separates out as a white precipitate, shown by the (s) — so this is also a precipitation reaction.
The other three equations each begin with a free metal (Zn, Fe or Pb) attacking a salt solution, which makes them displacement, not double displacement, reactions.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
• Both compounds exchange their ions — Ba²⁺ pairs with SO₄²⁻, Na⁺ pairs with Cl⁻.
• The coefficient 2 before NaCl balances the 2 Na and 2 Cl atoms.
• BaSO₄ is a white insoluble precipitate; it is also used as the "barium meal" in X-ray imaging of the stomach.
❌ Why the other options are incorrect
✗ Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) — a displacement reaction; the free metal zinc, being more reactive than copper, pushes Cu out of its salt. Only one exchange takes place.
✗ Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) — again displacement; iron displaces copper, and the blue solution turns light green as brown copper deposits on the iron.
✗ Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s) — displacement too; lead is above copper in the reactivity series, so it replaces copper. A free element is involved, so it cannot be double displacement.
📌 SSC & Railway Exam Facts
• Double displacement: AB + CD → AD + CB; displacement: A + BC → AC + B.
• A double displacement giving an insoluble solid is a precipitation reaction.
• AgNO₃ + NaCl → AgCl↓ (white) + NaNO₃ is the classic PYQ precipitation example.
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Combination reactions

Q76. CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat, this reaction is called:

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

Answer: (b) combination reaction

Correct Answer: Option 2 — combination reaction
In a combination (synthesis) reaction, two or more reactants join together to form a single product. Its general form is A + B → AB.
Here calcium oxide (CaO, quick lime) and water combine to give just one product, calcium hydroxide, Ca(OH)₂ (slaked lime) — two reactants, one product — so it is a textbook combination reaction.
The word "+ Heat" on the product side shows that heat is released, so the reaction is also exothermic. The process is called the slaking of lime; so much heat comes out that the water hisses and boils. A suspension of slaked lime in water is lime water, which turns milky with CO₂ — the standard test for carbon dioxide.
⚗️ The Reaction
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
• Two reactants → one product, so it is a combination reaction.
• Heat is written on the product side, so it is exothermic.
• Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O — this is why lime water turns milky.
❌ Why the other options are incorrect
✗ Wurtz reaction — an organic reaction in which two alkyl halides react with sodium in dry ether to give a higher alkane; it has nothing to do with lime and water.
✗ Wurtz fittig reaction — also organic, joining an aryl halide and an alkyl halide with sodium; again an organic coupling reaction, not an inorganic combination.
✗ decomposition reaction — there a single compound breaks up into two or more products, e.g. CaCO₃ → (heat) CaO + CO₂; here two reactants join, which is the exact opposite.
📌 SSC & Railway Exam Facts
• Combination: A + B → AB; decomposition: AB → A + B.
• CaO = quick lime, Ca(OH)₂ = slaked lime, CaCO₃ = limestone; Ca(OH)₂ solution = lime water.
• Lime water turns milky with CO₂ because insoluble CaCO₃ is formed.
• Exothermic: respiration, burning of fuels, slaking of lime; endothermic: photosynthesis, decomposition of CaCO₃.
• Decomposition by heat = thermal, by light = photolytic (2AgCl → 2Ag + Cl₂), by electricity = electrolytic.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Displacement reactions

Q77. Which of the following chemical reactions represents the displacement reaction?

  1. (a) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
  2. (b) 2AgCl(s) → 2Ag(s) + Cl₂(g)
  3. (c) CaO(s) + H₂O(l) → Ca(OH)₂(aq)
  4. (d) C(s) + O₂(g) → CO₂(g)
Answer & solution

Answer: (a) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Correct Answer: Option 1 — Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
In a displacement reaction, a more reactive element pushes out (displaces) a less reactive element from its compound. Its general form is A + BC → AC + B, and the giveaway is that one reactant is a free element and one is a compound.
In option 1, zinc is above copper in the reactivity series, so zinc displaces copper from copper sulphate. The blue colour of the CuSO₄ solution fades as it changes into colourless zinc sulphate, and reddish-brown copper is deposited on the zinc strip — the classic laboratory demonstration.
⚗️ The Reaction
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Zn is more reactive than Cu, so it takes the place of Cu in the sulphate.
• Zn is oxidised (Zn → Zn²⁺ + 2e⁻) and Cu²⁺ is reduced (Cu²⁺ + 2e⁻ → Cu) — a redox reaction as well.
• The blue solution turns colourless and brown copper settles out.
❌ Why the other options are incorrect
✗ 2AgCl(s) → 2Ag(s) + Cl₂(g) — a decomposition reaction; a single compound breaks into two substances in sunlight (photolytic decomposition), which is why silver chloride is used in photography and turns grey on standing in light.
✗ CaO(s) + H₂O(l) → Ca(OH)₂(aq) — a combination reaction; two reactants unite to give a single product, slaked lime, with heat released.
✗ C(s) + O₂(g) → CO₂(g) — also a combination reaction (and a combustion); carbon and oxygen join to form one product, with no element being displaced from a compound.
📌 SSC & Railway Exam Facts
• Displacement: A + BC → AC + B; a more reactive element displaces a less reactive one.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• Fe + CuSO₄ → FeSO₄ + Cu — blue solution turns light green; a favourite PYQ.
• Cu + ZnSO₄ → no reaction, because copper is less reactive than zinc.
• 2AgBr → (sunlight) 2Ag + Br₂ and 2AgCl → (sunlight) 2Ag + Cl₂ are photolytic decompositions.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Writing and balancing chemical equations

Q78. What is the value of x for balanced chemical equation 3Fe + xH₂O → Fe₃O₄ + 4H₂ ?

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

Answer: (d) 4

Correct Answer: Option 4 — 4
A chemical equation is balanced when the number of atoms of each element is the same on both sides, as required by the Law of Conservation of Mass — matter can neither be created nor destroyed in a chemical reaction.
⚗️ The Calculation
3Fe + xH₂O → Fe₃O₄ + 4H₂
• Iron (Fe): 3 on the left, 3 in Fe₃O₄ on the right — already balanced.
• Oxygen (O): left side has x O atoms; right side has 4 O atoms in Fe₃O₄ ⇒ x = 4.
• Hydrogen (H): left side has 2x = 2 × 4 = 8 H atoms; right side has 4 × 2 = 8 H atoms in 4H₂ — it checks out.
• The balanced equation is 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
This is the well-known reaction of iron with steam. Heated iron reacts with steam (not with cold water) to form iron(II, III) oxide, Fe₃O₄ (magnetic oxide of iron) and to liberate hydrogen gas. Note that only coefficients may be changed while balancing — a subscript inside a formula must never be altered.
❌ Why the other options are incorrect
✗ 1 — gives only 1 O and 2 H on the left against 4 O and 8 H on the right; the equation stays badly unbalanced.
✗ 5 — puts 5 O on the left but Fe₃O₄ needs only 4 O, leaving one oxygen atom unaccounted for.
✗ 3 — gives 3 O and 6 H on the left, short of the 4 O and 8 H required on the right.
📌 SSC & Railway Exam Facts
• Balancing is based on the Law of Conservation of Mass (Antoine Lavoisier).
• 3Fe + 4H₂O → Fe₃O₄ + 4H₂ — iron reacts with steam, not with cold water.
• Fe₃O₄ is the magnetic oxide of iron; its mineral form is magnetite.
• While balancing, change only the coefficients, never the subscripts in a formula.
• 2Mg + O₂ → 2MgO, 2H₂ + O₂ → 2H₂O and N₂ + 3H₂ ⇌ 2NH₃ are standard balanced equations.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Displacement reactions

Q79. When a more reactive metal is placed in a salt solution containing a less reactive metal, it replaces the less reactive metal. This reaction is known as _______.

  1. (a) Combination reaction
  2. (b) Displacement reaction
  3. (c) Decomposition reaction
  4. (d) Neutralisation reaction
Answer & solution

Answer: (b) Displacement reaction

Correct Answer: Option 2 — Displacement reaction
In a displacement reaction a more reactive element takes the place of a less reactive element in its compound, setting the less reactive one free. Its general pattern is A + BC → AC + B, and a free element must take part on the reactant side.
Whether one metal can push out another is decided entirely by the reactivity series. The higher a metal stands in the series, the stronger its tendency to lose electrons and form ions, so it can throw a lower metal out of its salt solution. A metal cannot displace a metal placed above it.
Dipping a strip of zinc into blue copper sulphate solution shows this beautifully: the blue colour fades and a reddish-brown deposit of copper collects on the zinc.
⚗️ The Reaction
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Zinc is above copper in the reactivity series, so it displaces copper from the sulphate.
• The blue copper sulphate solution turns colourless as ZnSO₄ is formed.
• The reverse, Cu + ZnSO₄, never happens — copper is the less reactive metal.
❌ Why the other options are incorrect
✗ Combination reaction — two or more substances join to give a single product, e.g. CaO + H₂O → Ca(OH)₂; here two separate products appear.
✗ Decomposition reaction — a single compound breaks up into simpler substances, e.g. 2H₂O → (electricity) 2H₂ + O₂; here two reactants are present.
✗ Neutralisation reaction — an acid reacts with a base to give salt and water; no free metal and no reactivity series is involved.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• A + BC → AC + B = displacement; AB + CD → AD + CB = double displacement.
• Fe + CuSO₄ → FeSO₄ + Cu (blue → pale green) is the classic PYQ example.
• Metals above hydrogen displace H₂ from dilute acids; Cu, Hg, Ag and Au do not.
• The thermite reaction, 2Al + Fe₂O₃ → 2Fe + Al₂O₃ + heat, is a highly exothermic displacement used to weld railway tracks.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Displacement reactions

Q80. In a displacement reaction, what is the key factor that determines whether a more reactive metal will displace a less reactive metal from its compound?

  1. (a) Temperature of the reaction
  2. (b) Size of the metal atoms
  3. (c) Position of the metals in the reactivity series
  4. (d) Concentration of the compound
Answer & solution

Answer: (c) Position of the metals in the reactivity series

Correct Answer: Option 3 — Position of the metals in the reactivity series
A displacement reaction follows the pattern A + BC → AC + B and takes place only when the free element A is more reactive than B. Whether that condition is met is read directly off the reactivity series (activity series) — the list of metals arranged in decreasing order of reactivity:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
A metal higher in the series loses electrons more readily, so it displaces any metal placed below it from that metal's salt solution. A metal lower in the series can never displace one above it, no matter how much the temperature or concentration is changed. So the relative position in the reactivity series is the deciding factor.
⚗️ The Reaction
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Zn is above Cu, so the reaction happens — the blue solution fades and reddish-brown copper is deposited.
• Cu(s) + ZnSO₄(aq) → no reaction, because Cu is below Zn.
• Every displacement reaction is also a redox reaction — the displacing metal is oxidised, the displaced metal ion is reduced.
❌ Why the other options are incorrect
✗ Temperature of the reaction — raising the temperature can make a reaction faster, but it cannot make a less reactive metal displace a more reactive one.
✗ Size of the metal atoms — atomic size influences reactivity indirectly through ionisation energy, but it is not the factor used to predict displacement.
✗ Concentration of the compound — concentration affects the rate of reaction only; a reaction that is impossible by the reactivity series stays impossible at any concentration.
📌 SSC & Railway Exam Facts
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• Potassium is the most reactive and gold the least reactive metal of the series.
• Metals above hydrogen displace H₂ from dilute acids; Cu, Hg, Ag, Au do not.
• Fe + CuSO₄ → FeSO₄ + Cu (blue → pale green) is the classic PYQ.
• The thermite reaction, 2Al + Fe₂O₃ → 2Fe + Al₂O₃, is a displacement used to weld railway tracks.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 1 · Combination reactions

Q81. Which of the following examples is NOT an example of combination reaction?

  1. (a) Formation of slaked lime from quick lime
  2. (b) Formation of water
  3. (c) Burning of coal
  4. (d) Formation of iron sulphate from copper sulphate and iron
Answer & solution

Answer: (d) Formation of iron sulphate from copper sulphate and iron

Correct Answer: Option 4 — Formation of iron sulphate from copper sulphate and iron
A combination reaction is one in which two or more substances join together to form a single product: A + B → AB. Its test is simple — count the products; a combination reaction has exactly one.
When iron is dropped into copper sulphate solution, iron does not combine with it. Being more reactive than copper, iron pushes copper out of its salt and takes its place, giving iron sulphate and free copper. Two products are formed and a free element is set free, so this is a displacement reaction, not a combination — hence it is the odd one out.
⚗️ The Reaction
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• Iron lies above copper in the reactivity series, so Fe displaces Cu²⁺.
• The blue colour of the copper sulphate solution fades to pale green (FeSO₄), and a reddish-brown deposit of copper forms on the iron nail.
• Two products and one free element → displacement, the classic PYQ example.
❌ Why the other options are incorrect
✗ Formation of slaked lime from quick lime — CaO(s) + H₂O(l) → Ca(OH)₂(aq); two reactants give one product, so it is a combination reaction (and a strongly exothermic one).
✗ Formation of water — 2H₂(g) + O₂(g) → 2H₂O(l); hydrogen and oxygen combine into the single product water, a textbook combination reaction.
✗ Burning of coal — C(s) + O₂(g) → CO₂(g); carbon and oxygen give the single product carbon dioxide, so it is a combination reaction (also a combustion/oxidation reaction).
📌 SSC & Railway Exam Facts
• Combination: A + B → AB; Decomposition: AB → A + B (the exact reverse).
• Quick lime = CaO, slaked lime = Ca(OH)₂, limestone/marble/chalk = CaCO₃.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• A reaction can belong to two classes at once — burning of coal is both combination and combustion.
• Ca(OH)₂ solution (lime water) turns milky with CO₂ because insoluble CaCO₃ is formed.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · Decomposition reactions (thermal, electrolytic, photolytic)

Q82. Which of the following compounds undergoes thermal decomposition to produce calcium oxide and carbon dioxide?

  1. (a) Calcium nitrate
  2. (b) Calcium chloride
  3. (c) Calcium carbonate
  4. (d) Calcium sulphate
Answer & solution

Answer: (c) Calcium carbonate

Correct Answer: Option 3 — Calcium carbonate
A decomposition reaction is one in which a single compound breaks down into two or more simpler substances. When the energy needed is supplied as heat, it is called thermal decomposition.
Calcium carbonate (CaCO₃) — found as limestone, marble and chalk — decomposes on strong heating at about 1000 °C to give calcium oxide (CaO) and carbon dioxide (CO₂). In industry this is carried out in a lime kiln, and the process is called calcination. The CaO produced is known as quick lime, and on adding water it gives slaked lime, Ca(OH)₂, with a great deal of heat.
Decomposition reactions are endothermic, because energy must be supplied to break the compound apart.
⚗️ The Reaction
CaCO₃(s) → (heat) CaO(s) + CO₂(g)
• One reactant gives two products — the signature of a decomposition reaction.
• CaO = quick lime; CaO + H₂O → Ca(OH)₂ (slaked lime) is a highly exothermic combination reaction.
• The CO₂ released turns lime water milky, forming a white precipitate of CaCO₃.
❌ Why the other options are incorrect
✗ Calcium nitrate — Ca(NO₃)₂ decomposes on heating to give CaO, nitrogen dioxide (NO₂) and oxygen, not carbon dioxide; it contains no carbon at all.
✗ Calcium chloride — CaCl₂ is a thermally stable salt; it melts rather than decomposing into CaO and CO₂.
✗ Calcium sulphate — CaSO₄ needs a very high temperature and then gives CaO and sulphur trioxide (SO₃); again there is no carbon in it.
📌 SSC & Railway Exam Facts
• CaCO₃ → (heat) CaO + CO₂ — the classic thermal decomposition / calcination reaction.
• Limestone, marble, chalk all have the same formula, CaCO₃.
• Quick lime = CaO, slaked lime = Ca(OH)₂, lime water = dilute Ca(OH)₂ solution.
• Other PYQ decompositions: 2FeSO₄ → (heat) Fe₂O₃ + SO₂ + SO₃, 2Pb(NO₃)₂ → (heat) 2PbO + 4NO₂ + O₂.
• Decomposition by heat = thermal, by light = photolytic (2AgCl → (sunlight) 2Ag + Cl₂), by electricity = electrolytic.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Double displacement and precipitation reactions

Q83. What are the balanced products of the following reaction?
Na₂SO₄(aq) + BaCl₂ (aq) → _____ + ______

  1. (a) 2BaSO₂ (s) + 2NaCl (aq) + 2O₂ (g)
  2. (b) BaSO₂ (s) + NaCl (aq) + O₂ (g)
  3. (c) BaSO₄ (s) + 2NaCl₂ (aq)
  4. (d) BaSO₄ (s) + 2NaCl (aq)
Answer & solution

Answer: (d) BaSO₄ (s) + 2NaCl (aq)

Correct Answer: Option 4 — BaSO₄ (s) + 2NaCl (aq)
This is a double displacement reaction of the pattern AB + CD → AD + CB: the two compounds simply exchange their ions. Ba²⁺ pairs with SO₄²⁻ to give barium sulphate, and Na⁺ pairs with Cl⁻ to give sodium chloride.
The radicals keep their own valencies, so the formulae must be BaSO₄ (both ions are divalent) and NaCl (both are monovalent). Since two Na⁺ ions are released by one Na₂SO₄, two NaCl are formed, which is why the coefficient 2 is needed before NaCl.
BaSO₄ is insoluble in water and separates out as a thick white precipitate — the (s) in the answer. A double displacement that gives an insoluble solid is also called a precipitation reaction.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
• Balance check — Na: 2 = 2, S: 1 = 1, O: 4 = 4, Ba: 1 = 1, Cl: 2 = 2. Fully balanced.
• Ba²⁺ + SO₄²⁻ → BaSO₄ (white precipitate); Na⁺ + Cl⁻ → NaCl (stays dissolved).
• No element changes its oxidation number, so this is not a redox reaction — only the partners change.
❌ Why the other options are incorrect
✗ 2BaSO₂ (s) + 2NaCl (aq) + 2O₂ (g) — wrong on two counts: the sulphate radical is SO₄²⁻, not SO₂, and no oxygen gas is released in a double displacement.
✗ BaSO₂ (s) + NaCl (aq) + O₂ (g) — again invents BaSO₂ and free O₂; it also leaves Na and Cl unbalanced (2 Na on the left, 1 on the right).
✗ BaSO₄ (s) + 2NaCl₂ (aq) — the product BaSO₄ is right, but NaCl₂ does not exist: Na⁺ has a valency of 1, so it can hold only one Cl⁻. This is the classic trap of blindly copying the subscript 2 from BaCl₂.
📌 SSC & Railway Exam Facts
• Double displacement: AB + CD → AD + CB; if an insoluble solid forms it is a precipitation reaction.
• White precipitates: BaSO₄, AgCl, PbSO₄, CaCO₃; yellow: PbI₂, AgBr.
• AgNO₃ + NaCl → AgCl↓ (white) + NaNO₃ is the other classic PYQ example.
• BaSO₄ is used as the barium meal for X-rays of the stomach, being insoluble and so non-toxic.
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Combination reactions

Q84. Calcium oxide reacts vigorously with water to produce slaked lime, releasing a large amount of heat. This reaction is an example of which of the following types of reaction?

  1. (a) Decomposition reaction
  2. (b) Double decomposition reaction
  3. (c) Precipitation reaction
  4. (d) Combination reaction
Answer & solution

Answer: (d) Combination reaction

Correct Answer: Option 4 — Combination reaction
A combination reaction is one in which two or more reactants join together to form a single product, in the general form A + B → AB.
Here calcium oxide (CaO), the quick lime of commerce, combines with water to give a single product, calcium hydroxide, Ca(OH)₂, known as slaked lime. Since two substances become one, it is a textbook combination reaction. As a large amount of heat is given out, it is also exothermic.
⚗️ The Reaction
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
• Two reactants → one product, so it is a combination reaction.
• Heat is released, so it is exothermic — the vessel becomes hot and the mixture hisses.
• A solution of Ca(OH)₂ is lime water; blowing CO₂ into it turns it milky, forming CaCO₃.
❌ Why the other options are incorrect
✗ Decomposition reaction — the opposite process, in which one compound splits into two or more, as in CaCO₃ → CaO + CO₂ (heat). Here, nothing splits.
✗ Double decomposition reaction — two compounds exchange their ions, AB + CD → AD + CB; here there is only one product, so no exchange happens.
✗ Precipitation reaction — this must give an insoluble solid that settles out; Ca(OH)₂ is only sparingly soluble but is formed as a solution/suspension of slaked lime, and the reaction is defined by the joining of two reactants.
📌 SSC & Railway Exam Facts
• Slaked lime is used to whitewash walls; Ca(OH)₂ + CO₂ → CaCO₃ + H₂O gives the shiny finish.
• Other combination reactions: 2Mg + O₂ → 2MgO, C + O₂ → CO₂, N₂ + 3H₂ → 2NH₃, SO₃ + H₂O → H₂SO₄.
• Quick lime — CaO, Slaked lime — Ca(OH)₂, Lime stone/Marble/Chalk — CaCO₃, Gypsum — CaSO₄·2H₂O.
• Exothermic — respiration, burning, combination reactions; Endothermic — photosynthesis, most decompositions.
• Decomposition is the exact reverse of a combination reaction.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Decomposition reactions (thermal, electrolytic, photolytic)

Q85. Which of the following compounds is NOT formed when ferrous sulphate is heated?

  1. (a) Sulphuric acid
  2. (b) Sulphur trioxide
  3. (c) Sulphur dioxide
  4. (d) Ferric oxide
Answer & solution

Answer: (a) Sulphuric acid

Correct Answer: Option 1 — Sulphuric acid
Ferrous sulphate crystals, FeSO₄·7H₂O, are green in colour. On heating, they first lose their water of crystallisation and turn white, and on stronger heating the anhydrous salt undergoes thermal decomposition.
⚗️ The Reaction
FeSO₄·7H₂O → (heat) FeSO₄ + 7H₂O
2FeSO₄ → (heat) Fe₂O₃ + SO₂ + SO₃
• The products are the reddish-brown solid Fe₂O₃ (ferric oxide) plus two gases, SO₂ and SO₃, which have a characteristic burning-sulphur smell.
• No sulphuric acid (H₂SO₄) is formed, because the decomposition takes place in the dry state with no water present to combine with SO₃.
• This is a thermal decomposition and an endothermic reaction.
Sulphuric acid would appear only if the SO₃ gas were later dissolved in water, SO₃ + H₂O → H₂SO₄ — a separate step that is not part of this heating.
❌ Why the other options are incorrect (they ARE formed)
✗ Sulphur trioxide — SO₃ is one of the two gases released; it is the anhydride of sulphuric acid.
✗ Sulphur dioxide — SO₂ is the other gas released; it has a pungent smell of burning sulphur and turns acidified KMnO₄ colourless.
✗ Ferric oxide — Fe₂O₃ is the reddish-brown solid residue left in the test tube, the colour change from green to brown being the visible sign of the reaction.
📌 SSC & Railway Exam Facts
• FeSO₄·7H₂O — green vitriol; CuSO₄·5H₂O — blue vitriol; ZnSO₄·7H₂O — white vitriol.
• H₂SO₄ is the "king of chemicals", made industrially by the Contact process.
• SO₂ and NO₂ dissolve in rainwater and cause acid rain.
• Other classic decompositions: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂, CaCO₃ → CaO + CO₂.
• Heating CuSO₄·5H₂O turns it from blue to white; adding water turns it blue again — a test for water.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Exothermic and endothermic reactions

Q86. Which of the following reactions is NOT an exothermic reaction?

  1. (a) Dilution of a strong acid
  2. (b) Photosynthesis
  3. (c) Burning of natural gas
  4. (d) Respiration
Answer & solution

Answer: (b) Photosynthesis

Correct Answer: Option 2 — Photosynthesis
An exothermic reaction releases heat to its surroundings, so the container feels warm; an endothermic reaction absorbs heat from its surroundings, so the container feels cold or the reaction needs a continuous supply of energy.
Photosynthesis is the classic endothermic reaction. Green plants absorb sunlight and use that energy, with chlorophyll, to combine carbon dioxide and water into glucose and oxygen. Energy is taken in and stored as chemical energy in the glucose, which is why the process stops in the dark. Every one of the other three options gives out heat.
⚗️ The Reaction
6CO₂ + 6H₂O → (sunlight, chlorophyll) C₆H₁₂O₆ + 6O₂ (endothermic)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (respiration, exothermic)
• Balance check for photosynthesis: 6 C, 12 H and 18 O on each side.
• Respiration is exactly the reverse of photosynthesis — it releases the energy that photosynthesis stored.
• Photosynthesis is also a combination reaction and requires light energy as an essential input.
❌ Why the other options are incorrect
✗ Dilution of a strong acid — adding conc. H₂SO₄ to water releases a large amount of heat; that is why acid is always added slowly to water, never water to acid, or the mixture may splash out.
✗ Burning of natural gas — the combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O + heat) is a strongly exothermic reaction; it is exactly how we cook food on an LPG/CNG stove.
✗ Respiration — the oxidation of glucose in our cells releases the energy that runs the body, making it a textbook exothermic reaction.
📌 SSC & Railway Exam Facts
• Exothermic — heat released (combustion, respiration, quick lime + water, acid dilution).
• Endothermic — heat absorbed (photosynthesis, decomposition of CaCO₃, electrolysis of water).
• CaO + H₂O → Ca(OH)₂ + heat is a standard exothermic combination reaction.
• CaCO₃ → (heat) CaO + CO₂ is a standard endothermic decomposition (thermal decomposition).
• Always add acid to water, never water to acid — the dilution of conc. H₂SO₄ is highly exothermic.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 2 · Double displacement and precipitation reactions

Q87. In which of the following types of reaction is there an exchange of ions between the reactants?

  1. (a) Decomposition
  2. (b) Addition reaction
  3. (c) Disproportion reaction
  4. (d) Double displacement reaction
Answer & solution

Answer: (d) Double displacement reaction

Correct Answer: Option 4 — Double displacement reaction
In a double displacement reaction two compounds dissolved in solution exchange their ions with one another, so that two completely new compounds are produced. Its general form is AB + CD → AD + CB.
Because the cations and anions simply swap partners, this reaction is also called an exchange reaction. When one of the new compounds is insoluble and settles out as a solid, the same reaction is additionally called a precipitation reaction.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
• Na⁺ pairs with Cl⁻ and Ba²⁺ pairs with SO₄²⁻ — the ions are exchanged between the two reactants.
• The white precipitate of barium sulphate (BaSO₄) is indicated by the state symbol (s).
❌ Why the other options are incorrect
✗ Decomposition — here a single compound breaks down into two or more simpler substances, AB → A + B, as in 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. There is only one reactant, so no exchange is possible.
✗ Addition reaction — an organic reaction in which an unsaturated compound adds atoms across its double or triple bond, e.g. C₂H₄ + H₂ → C₂H₆ (Ni catalyst). Nothing is swapped.
✗ Disproportion reaction — in this the same element is oxidised and reduced at the same time; it involves one species changing oxidation state, not two reactants trading ions.
📌 SSC & Railway Exam Facts
• Neutralisation, NaOH + HCl → NaCl + H₂O, is a double displacement reaction.
• AgNO₃ + NaCl → AgCl(s) + NaNO₃ gives a curdy white precipitate of silver chloride.
• Displacement: Fe + CuSO₄ → FeSO₄ + Cu — a more reactive metal displaces a less reactive one.
• Combination: CaO + H₂O → Ca(OH)₂ — quick lime to slaked lime, a highly exothermic change.
• Every precipitation reaction is a double displacement reaction, but not every double displacement reaction gives a precipitate.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 2 · Decomposition reactions (thermal, electrolytic, photolytic)

Q88. During the decomposition reaction of lead nitrate, the observed brown colour fumes are of which gas?

  1. (a) Ozone gas
  2. (b) Nitrogen gas
  3. (c) Nitrogen dioxide gas
  4. (d) Hydrogen gas
Answer & solution

Answer: (c) Nitrogen dioxide gas

Correct Answer: Option 3 — Nitrogen dioxide gas
When lead nitrate, Pb(NO₃)₂, is heated strongly in a test tube it undergoes thermal decomposition. The white crystals first crackle (this is why it is called a decrepitation reaction) and then give off dense brown fumes together with a yellow residue.
⚗️ The Reaction
2Pb(NO₃)₂ → (heat) 2PbO + 4NO₂ + O₂
• The brown fumes are of nitrogen dioxide (NO₂).
• The yellow solid left behind is lead(II) oxide, PbO, also called litharge.
• The colourless gas that escapes along with NO₂ is oxygen, which relights a glowing splinter.
• This is a thermal decomposition and an endothermic reaction.
❌ Why the other options are incorrect
✗ Ozone gas — O₃ is a pale blue gas with a pungent smell; it is formed in the stratosphere from O₂ by UV light, not from lead nitrate.
✗ Nitrogen gas — N₂ is colourless and odourless, makes up 78% of air, and is not a product of this decomposition.
✗ Hydrogen gas — H₂ is colourless; lead nitrate contains no hydrogen at all, so it cannot possibly be released.
📌 SSC & Railway Exam Facts
• Brown gas = NO₂, an acidic oxide; it dissolves in rainwater to give nitric acid and causes acid rain.
• Other classic decomposition reactions: CaCO₃ → CaO + CO₂ (heat), 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (heat), 2H₂O → 2H₂ + O₂ (electricity), 2AgCl → 2Ag + Cl₂ (sunlight).
• Decomposition by heat = thermolysis, by electricity = electrolysis, by light = photolysis.
• Silver salts decompose in sunlight, which is the basis of black-and-white photography.
• Most decomposition reactions are endothermic; the reverse of a combination reaction.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Oxidation, reduction and redox reactions

Q89. Which of the following is NOT an example of oxidation reaction?

  1. (a) Rancidity of oils
  2. (b) Corrosion
  3. (c) Formation of zinc from zinc oxide
  4. (d) Formation of magnesium oxide from magnesium
Answer & solution

Answer: (c) Formation of zinc from zinc oxide

Correct Answer: Option 3 — Formation of zinc from zinc oxide
Oxidation is the gain of oxygen or the loss of hydrogen or of electrons by a substance. Reduction is exactly the opposite — the loss of oxygen or the gain of hydrogen or of electrons.
When zinc oxide (ZnO) is converted into zinc (Zn), oxygen is removed from the compound. That is a loss of oxygen, so this change is a reduction, not an oxidation. It is therefore the required answer.
⚗️ The Reaction
ZnO + C → Zn + CO
• ZnO loses oxygen → it is reduced (ZnO is the oxidising agent).
• C gains oxygen → it is oxidised (carbon is the reducing agent).
• Both processes occur together, so the reaction is a redox reaction. This step is the smelting of zinc.
❌ Why the other options are incorrect (they ARE oxidations)
✗ Rancidity of oils — fats and oils are oxidised by the oxygen of air, giving the foul smell and taste of stale food. It is prevented by antioxidants and by flushing packets with nitrogen gas.
✗ Corrosion — a metal is slowly oxidised by air and moisture, as in the rusting of iron to hydrated iron(III) oxide, Fe₂O₃·xH₂O.
✗ Formation of magnesium oxide from magnesium — magnesium gains oxygen, 2Mg + O₂ → 2MgO, a textbook oxidation and combination reaction.
📌 SSC & Railway Exam Facts
• OIL RIG — Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
• The substance oxidised is the reducing agent; the one reduced is the oxidising agent.
• Rusting needs both air (oxygen) and water; it is prevented by painting, greasing, galvanisation and electroplating.
• Galvanisation is coating iron with a layer of zinc.
• Chips packets are filled with nitrogen to stop rancidity.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 1 · Oxidation, reduction and redox reactions

Q90. Which of the following is an example of reduction reaction?

  1. (a) Conversion of carbon into carbon dioxide
  2. (b) Conversion of copper oxide into copper
  3. (c) Conversion of ethanol into acetic acid
  4. (d) Conversion of copper into copper oxide
Answer & solution

Answer: (b) Conversion of copper oxide into copper

Correct Answer: Option 2 — Conversion of copper oxide into copper
Reduction is the removal of oxygen from a substance, or the addition of hydrogen to it; in electronic terms it is the gain of electrons. Oxidation is the exact opposite.
When copper oxide (CuO) becomes copper (Cu), the compound loses its oxygen — so copper oxide is reduced. This is done in the laboratory by passing hydrogen gas over heated CuO: the black CuO turns into reddish-brown copper.
⚗️ The Reaction
CuO + H₂ → (heat) Cu + H₂O
• CuO loses oxygen → it is reduced; H₂ is the reducing agent.
• H₂ gains oxygen → it is oxidised to water; CuO is the oxidising agent.
• Oxidation and reduction occur together, so this is a redox reaction.
❌ Why the other options are incorrect
✗ Conversion of carbon into carbon dioxide — C + O₂ → CO₂; carbon gains oxygen, so this is oxidation.
✗ Conversion of ethanol into acetic acid — C₂H₅OH → CH₃COOH with alkaline KMnO₄ or acidified K₂Cr₂O₇; oxygen is added, so it is oxidation.
✗ Conversion of copper into copper oxide — 2Cu + O₂ → 2CuO; copper gains oxygen and turns black, which is oxidation, the reverse of the answer.
📌 SSC & Railway Exam Facts
• Reduction = loss of oxygen / gain of hydrogen / gain of electrons; remember OIL RIG.
• CuO + H₂ → Cu + H₂O is the standard NCERT redox example — black to reddish-brown.
• The oxidising agent is itself reduced; the reducing agent is itself oxidised.
• Ethanol → ethanoic (acetic) acid by alkaline KMnO₄ or acidified K₂Cr₂O₇ — called oxidising agents.
• Rusting and rancidity are everyday oxidation processes; rancidity is checked by antioxidants.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 2 · Decomposition reactions (thermal, electrolytic, photolytic)

Q91. When a decomposition reaction of Limestone is carried out by heating, it is called ______.

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

Answer: (d) thermal decomposition

Correct Answer: Option 4 — thermal decomposition
A decomposition reaction is one in which a single compound breaks up into two or more simpler substances. When the energy required for this break-up is supplied in the form of heat, the reaction is called thermal decomposition.
⚗️ The Reaction
CaCO₃(s) → (heat) CaO(s) + CO₂(g)
• Limestone (calcium carbonate, CaCO₃), heated to about 1000 °C, gives quick lime (calcium oxide, CaO) and carbon dioxide.
• One reactant → two products, so it is a decomposition; the energy comes from heat, so it is thermal decomposition.
• The reaction absorbs heat, so it is also an endothermic reaction. It is used industrially to make cement, glass and quick lime.
❌ Why the other options are incorrect
✗ double displacement reaction — two compounds exchange their ions, e.g. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq); here there is only one reactant.
✗ combination reaction — two or more substances join to form a single product, e.g. CaO + H₂O → Ca(OH)₂; that is the exact opposite of decomposition.
✗ exothermic reaction — a reaction that releases heat. Limestone decomposition absorbs heat, so it is endothermic, not exothermic.
📌 SSC & Railway Exam Facts
• Decomposition by heat = thermal, by light = photolytic, by electricity = electrolytic.
• 2AgBr → (sunlight) 2Ag + Br₂ — photolytic decomposition, used in black-and-white photography.
• 2H₂O → (electricity) 2H₂ + O₂ — electrolysis of water; H₂ : O₂ = 2 : 1 by volume.
• CaO + H₂O → Ca(OH)₂ (slaked lime) is a vigorous exothermic reaction.
• Decomposition reactions are the opposite of combination reactions and are generally endothermic.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Combination reactions

Q92. Burning of coal is an example of which type of chemical reaction?

  1. (a) Combination reaction
  2. (b) Decomposition reaction
  3. (c) Double decomposition reaction
  4. (d) Reduction reaction
Answer & solution

Answer: (a) Combination reaction

Correct Answer: Option 1 — Combination reaction
Coal is essentially carbon. When it burns, the carbon combines with the oxygen of air to form a single product, carbon dioxide.
⚗️ The Reaction
C(s) + O₂(g) → CO₂(g) + heat
• Two reactants join to form one single product, which is the definition of a combination (synthesis) reaction.
• Heat and light are given out, so it is also combustion, an oxidation and an exothermic reaction.
• Carbon gains oxygen, so carbon is oxidised — it is not a reduction.
A combination reaction can be between two elements (2Mg + O₂ → 2MgO), two compounds (CaO + H₂O → Ca(OH)₂), or an element and a compound (2CO + O₂ → 2CO₂).
❌ Why the other options are incorrect
✗ Decomposition reaction — here a single compound splits into two or more products (CaCO₃ → CaO + CO₂); burning coal is the opposite, two things joining into one.
✗ Double decomposition reaction — another name for a double displacement reaction, in which two compounds exchange ions (AgNO₃ + NaCl → AgCl + NaNO₃); no exchange of ions occurs here.
✗ Reduction reaction — reduction means loss of oxygen or gain of hydrogen; in burning coal, carbon gains oxygen, so it is oxidation, not reduction.
📌 SSC & Railway Exam Facts
• Combination: A + B → AB; Decomposition: AB → A + B.
• CaO + H₂O → Ca(OH)₂ is a combination reaction that is strongly exothermic.
• Burning of coal, natural gas, candle and respiration are all exothermic processes.
• Incomplete combustion of coal gives the poisonous gas carbon monoxide (CO).
• Coal varieties in increasing order of carbon: peat < lignite < bituminous < anthracite.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Double displacement and precipitation reactions

Q93. Which of the following is produced when barium chloride reacts with sodium sulphate?

  1. (a) Black precipitate of BaSO₄
  2. (b) White precipitate of BaSO₄
  3. (c) Yellow precipitate of BaSO₄
  4. (d) Brown precipitate of BaSO₄
Answer & solution

Answer: (b) White precipitate of BaSO₄

Correct Answer: Option 2 — White precipitate of BaSO₄
When solutions of barium chloride and sodium sulphate are mixed, the two compounds exchange their ions — Ba²⁺ pairs with SO₄²⁻ and Na⁺ pairs with Cl⁻. Barium sulphate is insoluble in water, so it separates at once as a thick white precipitate.
⚗️ The Reaction
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
• The (s) marks BaSO₄ as the insoluble white precipitate.
• Both compounds exchange ions, so it is a double displacement reaction; because a precipitate forms it is also called a precipitation reaction.
• Atoms balance: 1 Ba, 2 Cl, 2 Na, 1 S, 4 O on each side.
This reaction is the standard laboratory test for the sulphate ion (SO₄²⁻) — a white precipitate that is insoluble even in dilute HCl.
❌ Why the other options are incorrect
✗ Black precipitate of BaSO₄ — black precipitates are typical of sulphides such as PbS and CuS, not of barium sulphate.
✗ Yellow precipitate of BaSO₄ — yellow precipitates are given by PbI₂ and AgI; BaSO₄ is never yellow.
✗ Brown precipitate of BaSO₄ — brown is the colour of Fe(OH)₃ or rust; barium sulphate is pure white.
📌 SSC & Railway Exam Facts
• BaSO₄ — white; AgCl — white; PbI₂ — yellow; Cu(OH)₂ — blue; Fe(OH)₃ — reddish brown; PbS — black.
• BaSO₄ is used as the "barium meal" in X-ray imaging of the stomach and intestines.
• Double displacement: AB + CD → AD + CB.
• Test for sulphate — white ppt with BaCl₂, insoluble in dilute HCl; test for chloride — white ppt with AgNO₃.
• A precipitation reaction always produces an insoluble salt, shown by (s) in the equation.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Double displacement and precipitation reactions

Q94. The following equation is of which type of reaction?
CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄

  1. (a) Combination reaction
  2. (b) Combustion reaction
  3. (c) Neutralisation reaction
  4. (d) Double displacement reaction
Answer & solution

Answer: (d) Double displacement reaction

Correct Answer: Option 4 — Double displacement reaction
In a double displacement reaction two compounds exchange their ions to form two new compounds: AB + CD → AD + CB.
⚗️ The Reaction
CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)
• Cu²⁺ leaves the sulphate and pairs with OH⁻; Na⁺ pairs with SO₄²⁻ — a clear exchange of ions.
• Copper hydroxide, Cu(OH)₂, is insoluble and separates as a blue precipitate (s), so this is also a precipitation reaction.
• The equation balances: 1 Cu, 1 S, 6 O, 2 Na, 2 H on each side.
The blue colour of copper sulphate solution fades as the blue gelatinous precipitate of copper hydroxide settles out.
❌ Why the other options are incorrect
✗ Combination reaction — two or more reactants would have to give one single product (CaO + H₂O → Ca(OH)₂); here there are two products.
✗ Combustion reaction — burning in oxygen with the release of heat and light; there is no oxygen gas and no burning here.
✗ Neutralisation reaction — needs an acid and a base giving salt + water; CuSO₄ is a salt, not an acid, and no water is formed.
📌 SSC & Railway Exam Facts
• Double displacement: AB + CD → AD + CB; often forms a precipitate.
• Na₂SO₄ + BaCl₂ → BaSO₄(s) + 2NaCl — classic precipitation example.
• Precipitate colours: Cu(OH)₂ blue, Fe(OH)₃ reddish brown, AgCl white, PbI₂ yellow.
• Displacement: Fe + CuSO₄ → FeSO₄ + Cu — only one element is displaced.
• CuSO₄·5H₂O is blue vitriol; on heating it turns white as it loses water of crystallisation.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Decomposition reactions (thermal, electrolytic, photolytic)

Q95. Ferrous sulphate decomposes to which of the following substances?

  1. (a) Ferric oxide (Fe₂O₃) only
  2. (b) Ferric oxide (Fe₂O₃) and sulphur dioxide (SO₂) only
  3. (c) Ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃)
  4. (d) Ferric oxide (Fe₂O₃) and sulphur trioxide (SO₃) only
Answer & solution

Answer: (c) Ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃)

Correct Answer: Option 3 — Ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃)
This is the standard thermal decomposition reaction of the NCERT syllabus. Ferrous sulphate crystals, FeSO₄·7H₂O, are light green. On heating they first lose their water of crystallisation and turn into white anhydrous FeSO₄; on stronger heating the salt decomposes completely.
⚗️ The Reaction
2FeSO₄(s) → (heat) Fe₂O₃(s) + SO₂(g) + SO₃(g)
• The residue left behind is Fe₂O₃, reddish brown ferric oxide.
• Two gases with the choking smell of burning sulphur are released — SO₂ and SO₃.
• A coefficient of 2 is needed before FeSO₄, because one Fe₂O₃ unit contains two iron atoms.
• Before this step: FeSO₄·7H₂O → (heat) FeSO₄ + 7H₂O, the green crystals turning white.
❌ Why the other options are incorrect
✗ Ferric oxide (Fe₂O₃) only — incomplete; the sulphate radical also decomposes, and it must give off gaseous oxides of sulphur.
✗ Ferric oxide (Fe₂O₃) and sulphur dioxide (SO₂) only — the most common half-answer; it leaves out SO₃, which is produced as well.
✗ Ferric oxide (Fe₂O₃) and sulphur trioxide (SO₃) only — again incomplete, because SO₂ is also among the products.
📌 SSC & Railway Exam Facts
• Green vitriol = FeSO₄·7H₂O; blue vitriol = CuSO₄·5H₂O; white vitriol = ZnSO₄·7H₂O.
• Decomposition is an endothermic reaction; it may be thermal (heat), electrolytic (electricity) or photochemical (light).
• CaCO₃ → (heat) CaO + CO₂ and 2Pb(NO₃)₂ → (heat) 2PbO + 4NO₂ + O₂ (brown fumes) are the other standard decompositions.
• 2H₂O → (electricity) 2H₂ + O₂ is electrolytic; 2AgCl → (sunlight) 2Ag + Cl₂ is photochemical decomposition.
• SO₂ dissolves in rain water to cause acid rain; Fe₂O₃ is the iron ore haematite.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Displacement reactions

Q96. The chemical reaction Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) represents which type of reaction?

  1. (a) Combination reaction
  2. (b) Combustion reaction
  3. (c) Displacement reaction
  4. (d) Double Displacement reaction
Answer & solution

Answer: (c) Displacement reaction

Correct Answer: Option 3 — Displacement reaction
In a displacement reaction a more reactive element pushes out (displaces) a less reactive element from its compound. Its general form is A + BC → AC + B.
Here iron (Fe) is more reactive than copper (Cu) — in the reactivity series Fe lies above Cu — so iron displaces copper from copper sulphate solution. The blue colour of the CuSO₄ solution slowly fades to the light green of FeSO₄, and a reddish-brown deposit of copper appears on the iron.
⚗️ The Reaction
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• Fe stands above Cu in the reactivity series, so it takes the sulphate radical away from copper.
• Blue CuSO₄ solution turns into light green FeSO₄ solution — the visible proof of the reaction.
• It is also a redox reaction: Fe → Fe²⁺ + 2e⁻ (oxidation) and Cu²⁺ + 2e⁻ → Cu (reduction).
❌ Why the other options are incorrect
✗ Combination reaction — there two or more substances join to give a single product, e.g. CaO + H₂O → Ca(OH)₂; here two separate products are formed.
✗ Combustion reaction — a substance burns in oxygen, giving out heat and light; in this reaction there is no oxygen and no burning at all.
✗ Double Displacement reaction — that needs two compounds to exchange their ions (AB + CD → AD + CB), e.g. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. Here one reactant is a free element (Fe), so only a single displacement takes place.
📌 SSC & Railway Exam Facts
• Displacement: A + BC → AC + B; double displacement: AB + CD → AD + CB.
• Fe + CuSO₄ → FeSO₄ + Cu and Zn + CuSO₄ → ZnSO₄ + Cu are the standard PYQ examples.
• Cu + FeSO₄ → no reaction, because copper is less reactive than iron.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au.
• An iron vessel must not be used to store copper sulphate solution, because the iron is eaten away by this very displacement reaction.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 2 · Double displacement and precipitation reactions

Q97. Which of the following is/are an example of a double displacement reaction?
(1) C (s) + O₂ (g) → CO₂ (g)
(2) Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl(aq)
(3) Pb(s) + CuCl₂ (aq) → PbCl₂ (aq) + Cu(s)
(4) CaCO₃ (s) → CaO (s) + CO₂ (g)

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

Answer: (c) Only 2

Correct Answer: Option 3 — Only 2
In a double displacement reaction two compounds exchange their ions to give two new compounds. The general form is AB + CD → AD + CB. When one product is an insoluble solid, the reaction is also called a precipitation reaction.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
• Na⁺ pairs with Cl⁻ and Ba²⁺ pairs with SO₄²⁻ — a clear exchange of ions.
• The white precipitate of barium sulphate (BaSO₄), shown by the (s), makes it a precipitation reaction as well.
Only reaction (2) shows such an exchange, so "Only 2" is the answer.
❌ Why the other options are incorrect
✗ Only 2 and 3 — reaction (3), Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s), is a single displacement reaction: the more reactive lead displaces copper; only one element is displaced, not two ions exchanged.
✗ Only 1, 2 and 3 — reaction (1), C + O₂ → CO₂, is a combination (and combustion) reaction, two substances forming one product.
✗ Only 1 and 4 — reaction (4), CaCO₃ → CaO + CO₂, is thermal decomposition, and (1) is combination; neither involves any exchange of ions.
📌 SSC & Railway Exam Facts
• Double displacement: AB + CD → AD + CB; neutralisation is a special case of it.
• BaSO₄ and AgCl are classic white precipitates; PbI₂ gives a yellow precipitate.
• Na₂SO₄ + BaCl₂ and Pb(NO₃)₂ + KI are the two standard NCERT precipitation examples.
• CaCO₃ → CaO + CO₂ is decomposition; CaO is quick lime, used to make cement.
• In a displacement reaction a more reactive metal displaces a less reactive one from its salt solution.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 2 · Corrosion and rancidity

Q98. Silver ornaments become black after some time because silver reacts with ______ present in the air.

  1. (a) sulphur
  2. (b) oxygen
  3. (c) carbon dioxide
  4. (d) nitrogen
Answer & solution

Answer: (a) sulphur

Correct Answer: Option 1 — sulphur
When a metal is slowly eaten away by the gases and moisture of the air, the process is called corrosion. In the case of silver (Ag) it is given the special name tarnishing.
Air contains small amounts of sulphur compounds, chiefly hydrogen sulphide (H₂S). Silver reacts with this sulphur to form a thin coating of silver sulphide (Ag₂S), which is black. That is why silver ornaments, spoons and coins lose their shine and turn blackish on standing. Cleaning them with a paste or an electrochemical bath removes the Ag₂S layer and restores the shine.
⚗️ The Reaction
2Ag + H₂S → Ag₂S + H₂
• The product Ag₂S (silver sulphide) is the black layer seen on the ornament.
• The same idea explains copper turning green (basic copper carbonate) and iron rusting (hydrated iron oxide).
❌ Why the other options are incorrect
✗ oxygen — silver is a noble, less reactive metal and does not form an oxide layer with atmospheric oxygen at room temperature.
✗ carbon dioxide — CO₂ with moisture turns copper green, not silver black.
✗ nitrogen — atmospheric N₂ is highly inert because of its triple bond and does not react with silver.
📌 SSC & Railway Exam Facts
• Silver tarnishing: Ag₂S, black; copper corrosion: CuCO₃·Cu(OH)₂, green.
• Rusting of iron: Fe₂O₃·xH₂O, reddish-brown; it needs both air (O₂) and water.
• Rusting is prevented by painting, greasing, galvanisation (zinc coating) and electroplating.
• Galvanisation protects iron even if the zinc coating is scratched, because zinc is more reactive.
• Silver is the best conductor of electricity, followed by copper and gold.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 2 · Decomposition reactions (thermal, electrolytic, photolytic)

Q99. Which of the following reactions is an example of a decomposition reaction?

  1. (a) Silver chloride turns grey in sunlight
  2. (b) Burning of coal
  3. (c) Formation of water from H₂(g) and O₂(g)
  4. (d) Burning of natural gas
Answer & solution

Answer: (a) Silver chloride turns grey in sunlight

Correct Answer: Option 1 — Silver chloride turns grey in sunlight
In a decomposition reaction a single compound breaks down into two or more simpler substances, the general form being AB → A + B. Energy must be supplied, and according to its source the reaction is called thermal decomposition (heat), photolytic decomposition (light) or electrolytic decomposition (electricity).
⚗️ The Reaction
2AgCl(s) → (sunlight) 2Ag(s) + Cl₂(g)
• White silver chloride breaks up into grey metallic silver and chlorine gas, so the solid turns grey.
• Because sunlight supplies the energy, it is a photolytic (photochemical) decomposition — the same reaction that is used in black-and-white photography.
❌ Why the other options are incorrect
✗ Burning of coal — C + O₂ → CO₂ is a combination reaction (and combustion); two reactants give one product, the opposite of decomposition.
✗ Formation of water from H₂(g) and O₂(g) — 2H₂ + O₂ → 2H₂O is again a combination reaction; water being *split* would be decomposition.
✗ Burning of natural gas — CH₄ + 2O₂ → CO₂ + 2H₂O is an oxidation / combustion reaction, giving out heat and light, not a breakdown into simpler elements.
📌 SSC & Railway Exam Facts
• Decomposition: AB → A + B; it is the exact opposite of a combination reaction.
• Thermal: CaCO₃ → CaO + CO₂; 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (brown fumes).
• Photolytic: 2AgCl → 2Ag + Cl₂ (grey) and 2AgBr → 2Ag + Br₂ — used in photography.
• Electrolytic: 2H₂O → 2H₂ + O₂, giving hydrogen at the cathode and oxygen at the anode in a 2 : 1 volume ratio.
• Most decomposition reactions are endothermic; most combination reactions are exothermic.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 3 · Displacement reactions

Q100. A student took iron nails and dipped them in solution 'X' for twenty minutes. After some time he noticed that the colour of the iron nails became brownish.
Identify solution 'X' in which the nail was dipped.

  1. (a) Zinc sulphate
  2. (b) Lead sulphate
  3. (c) Copper sulphate
  4. (d) Iron sulphate
Answer & solution

Answer: (c) Copper sulphate

Correct Answer: Option 3 — Copper sulphate
This is the classic NCERT activity on the displacement reaction. Iron (Fe) lies above copper (Cu) in the reactivity series, so iron is more reactive and displaces copper from a solution of copper sulphate (CuSO₄).
⚗️ The Reaction
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• The displaced copper deposits on the nail as a reddish-brown layer, which is exactly the "brownish" colour observed.
• The solution changes from blue (CuSO₄) to light green (FeSO₄).
• It is also a redox reaction: Fe is oxidised (loses electrons) and Cu²⁺ is reduced.
❌ Why the other options are incorrect
✗ Zinc sulphate — zinc is more reactive than iron, so iron cannot displace zinc; no deposit and no colour change occurs.
✗ Lead sulphate — lead sulphate (PbSO₄) is insoluble in water, so no such solution reaction is possible; besides, lead would deposit greyish, not brown.
✗ Iron sulphate — the nail and the solution contain the same metal, so there is nothing to displace; no reaction at all.
📌 SSC & Railway Exam Facts
• Fe + CuSO₄ → FeSO₄ + Cu: blue solution turns green, nail turns brown.
• A more reactive metal displaces a less reactive metal from its salt solution.
• Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag: colourless solution turns blue, silver deposits.
• CuSO₄·5H₂O is blue vitriol; FeSO₄·7H₂O is green vitriol; ZnSO₄·7H₂O is white vitriol.
• In redox, oxidation = loss of electrons (OIL), reduction = gain of electrons (RIG).
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