RRB Technician Grade III 2024-25 27 December 2024 Shift 3 GK Questions – with answers & solutions

Correct Answer: Option 1 — Decrease in the amount of ozone in the atmosphere is due to chlorofluorocarbons (CFC).
Ozone (O₃) is a molecule of three atoms of oxygen. High up in the stratosphere it forms a shield that absorbs harmful ultraviolet (UV) radiation coming from the Sun and prevents it from reaching the Earth's surface.
CFCs used in refrigerators, air-conditioners and aerosol sprays drift up to the stratosphere. UV light breaks them and releases chlorine atoms, and a single chlorine atom destroys thousands of ozone molecules in a chain reaction. This thinning of the layer is the ozone hole, first reported over Antarctica.
❌ Why the other options are incorrect
✗ It is formed by combining only two atoms of oxygen — O₂ has two atoms; ozone has three (O₃), made when UV splits O₂ and the free O joins another O₂.
✗ It is not poisonous — ozone is a deadly poison at ground level; it is useful only in the upper atmosphere.
✗ It causes blood cancer — ozone does not cause blood cancer; the UV rays it blocks cause skin cancer and cataract.
📌 SSC & Railway Exam Facts
• Ozone layer lies in the stratosphere, about 15–35 km above the Earth.
• The ozone hole over Antarctica was reported in 1985.
• The Montreal Protocol (1987) froze and phased out CFC production.
• 16 September is observed as the World Ozone Day.
• Ozone thickness is measured in Dobson Units (DU).

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

Correct Answer: Option 4 — homologous series
A homologous series is a family of organic compounds in which the same functional group replaces hydrogen in a carbon chain, so that all the members share one general formula and similar chemical properties. Successive members differ from one another by a single –CH₂– unit, that is, by 14 u of molecular mass.
For example, replacing one hydrogen of an alkane by the –OH group builds the alcohol series, CₙH₂ₙ₊₁OH: CH₃OH (methanol) → C₂H₅OH (ethanol) → C₃H₇OH (propanol). In the same way the –COOH group builds the carboxylic acid series, and –CHO the aldehyde series.
Within a series, the chemical properties stay similar because the reacting part — the functional group — is unchanged, while the physical properties (melting point, boiling point, density) change gradually as the chain lengthens and the molecular mass rises.
❌ Why the other options are incorrect
✗ isobars — a term from atomic structure, not organic chemistry: atoms of different elements with different atomic numbers but the same mass number, e.g. ₂₀Ca⁴⁰ and ₁₈Ar⁴⁰.
✗ isochores — a term from physics/thermodynamics for a process at constant volume; it has nothing to do with carbon chains.
✗ isomers — compounds with the same molecular formula but different structures, e.g. n-butane and isobutane (C₄H₁₀); they are a single pair of compounds, not a series built by one functional group.
📌 SSC & Railway Exam Facts
• Successive homologues differ by –CH₂–, i.e. by 14 u of molecular mass.
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂, alcohol CₙH₂ₙ₊₁OH.
• In a homologous series chemical properties are similar, physical properties change gradually.
• Functional groups: –OH alcohol, –CHO aldehyde, >C=O ketone, –COOH carboxylic acid.
• Isomers = same formula, different structure; homologues = different formula, same functional group — a classic PYQ contrast.

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

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

Correct Answer: Option 4 — Only statement I is correct.
Statement I is correct. A saturated carbon compound is one in which the carbon atoms are joined only by single covalent bonds (C–C), so every carbon carries the maximum number of hydrogen atoms it can. These are the alkanes, CₙH₂ₙ₊₂ — CH₄, C₂H₆, C₃H₈ and so on.
Statement II is incorrect. An unsaturated compound is defined by exactly the opposite feature — it contains at least one carbon–carbon double bond (C=C) or triple bond (C≡C). These are the alkenes, CₙH₂ₙ (e.g. ethene, C₂H₄) and the alkynes, CₙH₂ₙ₋₂ (e.g. ethyne, C₂H₂). The statement has simply repeated the definition of a saturated compound.
The two families are told apart experimentally: unsaturated compounds decolourise bromine water and give addition reactions with hydrogen over Ni/Pd/Pt, whereas saturated compounds do neither and instead give substitution reactions. Saturated hydrocarbons also burn with a clean blue flame, while unsaturated ones burn with a yellow, sooty flame.
❌ Why the other options are incorrect
✗ Neither statement I nor II is correct — statement I is a textbook-correct definition of a saturated compound, so this cannot be right.
✗ Both statements I and II are correct — they cannot both be true, since they give the same definition to two opposite terms.
✗ Only statement II is correct — statement II is precisely the one that is wrong; unsaturated compounds need a double or triple bond.
📌 SSC & Railway Exam Facts
• Saturated = single bonds only (alkanes); unsaturated = double or triple bond (alkenes, alkynes).
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Bromine water test: unsaturated compounds decolourise the brown-orange colour; alkanes do not.
• Alkanes burn with a blue flame; unsaturated hydrocarbons give a yellow, sooty flame.
• Alkanes react by substitution; alkenes and alkynes react by addition (hydrogenation over Ni).

Correct Answer: Option 3 — Only statement I is correct.
Statement I is correct. The atomic number (Z) of an element is defined as the number of protons present in the nucleus of its atom. Because an atom as a whole is electrically neutral, the number of electrons also equals Z. The atomic number is the true identity card of an element — change it and you get a different element altogether (Z = 1 hydrogen, Z = 6 carbon, Z = 8 oxygen, Z = 11 sodium). Elements in the modern periodic table are arranged in increasing order of atomic number, as stated by the Modern Periodic Law (Moseley, 1913).
Statement II is incorrect. The number of neutrons is not the atomic number. Neutrons are counted through the mass number (A) = protons + neutrons, so number of neutrons = A − Z. Atoms of the same element can have different numbers of neutrons — those are its isotopes, such as ¹H, ²H and ³H, all of which still have Z = 1.
❌ Why the other options are incorrect
✗ Both statements I and II are correct — statement II is wrong; neutrons are obtained from A − Z, not from Z itself.
✗ Only statement II is correct — this is the exact reverse of the truth; the atomic number counts protons, never neutrons.
✗ Neither statement I nor II is correct — statement I is the standard definition of atomic number, so this option cannot be right.
📌 SSC & Railway Exam Facts
• Atomic number (Z) = number of protons = number of electrons in a neutral atom.
• Mass number (A) = protons + neutrons; neutrons = A − Z.
• Modern periodic law: properties are a periodic function of the atomic number — given by Moseley.
• Isotopes — same Z, different A; isobars — different Z, same A; isotones — same number of neutrons.
• Discoverers: proton — Goldstein/Rutherford, neutron — James Chadwick (1932), electron — J. J. Thomson.

Correct Answer: Option 2 — Both Zn and CO
This is the reduction step of zinc extraction. After roasting zinc sulphide ore (or calcining zinc carbonate ore) the metal is obtained as zinc oxide, ZnO. This oxide is then reduced to the metal by heating it with carbon (coke), which acts as the reducing agent.
Carbon has a greater affinity for oxygen than zinc has, so it takes the oxygen away from ZnO. The zinc is thereby reduced to free metal, while the carbon is oxidised to carbon monoxide (CO). Because both oxidation and reduction happen together, this is a redox reaction, and it is also a displacement reaction — carbon displaces zinc from its oxide.
Carbon reduction works only for the oxides of moderately reactive metals such as Zn, Fe, Pb and Cu. The highly reactive metals — K, Na, Ca, Mg and Al — cannot be reduced by carbon and must be obtained by the electrolysis of their molten salts.
⚗️ The Reaction
ZnO(s) + C(s) → (heat) Zn(s) + CO(g)
• ZnO is reduced (loses oxygen) to Zn; carbon is oxidised (gains oxygen) to CO.
• Count the atoms: 1 Zn, 1 C and 1 O on each side — the equation is balanced as written.
• With an excess of oxide the carbon can be taken all the way to CO₂, but the standard NCERT equation for zinc gives CO.
❌ Why the other options are incorrect
✗ Only Zn — the carbon cannot simply vanish; the oxygen it removes must leave as a carbon oxide, so a gaseous product is compulsory.
✗ Only CO — the whole purpose of the step is to obtain the metal; zinc must appear among the products.
✗ Zn and CO₂ — the standard equation for the reduction of ZnO by carbon gives CO, not CO₂; CO₂ would require ZnO + C in a 2 : 1 ratio (2ZnO + C → 2Zn + CO₂).
📌 SSC & Railway Exam Facts
• Roasting — heating a sulphide ore in excess air; calcination — heating a carbonate ore in limited air.
• 2ZnS + 3O₂ → 2ZnO + 2SO₂ (roasting); ZnCO₃ → (heat) ZnO + CO₂ (calcination).
• Carbon reduction works for Zn, Fe, Pb, Cu; K, Na, Ca, Mg, Al need electrolytic reduction.
• In the thermite reaction, Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat, used for welding railway tracks.
• Zinc's chief ores: zinc blende (ZnS), calamine (ZnCO₃) and zincite (ZnO).

Correct Answer: Option 3 — 12 u
The carbon-12 isotope (₆C¹²) has 6 protons and 6 neutrons, so its mass number is 6 + 6 = 12 and its atomic mass is exactly 12 u. This is not an approximation — C-12 is the modern standard of atomic mass itself.
Since 1961, the atomic mass unit (u), also written amu or dalton (Da), has been defined as exactly one-twelfth (1/12) of the mass of one atom of carbon-12. Every other atomic mass in the periodic table is quoted relative to this standard: an oxygen atom is 16 u because it is 16 times as heavy as 1/12 of a C-12 atom.
Note that the atomic mass of carbon printed in the periodic table is 12.011 u, because natural carbon is a mixture of the isotopes C-12 (about 98.9%) and C-13 (about 1.1%), with traces of C-14. The value 12 u exactly applies to the C-12 isotope alone.
❌ Why the other options are incorrect
✗ 14 u — this is the mass number of carbon-14, the radioactive isotope used in carbon dating; it is also the atomic mass of nitrogen.
✗ 16 u — this is the atomic mass of oxygen (₈O¹⁶), not of carbon.
✗ 6 u — 6 is the atomic number of carbon (its number of protons), not its atomic mass; this is the classic confusion between Z and A.
📌 SSC & Railway Exam Facts
• 1 u = 1/12 of the mass of one carbon-12 atom — C-12 is the standard for atomic masses.
• Carbon: Z = 6; isotopes C-12, C-13, C-14; the periodic-table value is 12.011 u.
• C-14 is radioactive with a half-life of about 5730 years and is used for carbon dating.
• Mass of proton ≈ mass of neutron ≈ 1 u; mass of electron ≈ 1/1836 u (negligible).
• Mass number (A) = protons + neutrons; atomic number (Z) = protons.

Correct Answer: Option 3 — Mn
Nitric acid (HNO₃) is a strong oxidising agent. When a metal reacts with it, the hydrogen that is set free is immediately oxidised by the acid itself to water, and the nitric acid is reduced to an oxide of nitrogen (N₂O, NO or NO₂). That is why HNO₃ normally does not give hydrogen gas with metals — the standard exception among all the common acids.
There are just two exceptions to memorise, and NCERT states them plainly: magnesium (Mg) and manganese (Mn) react with very dilute nitric acid to evolve hydrogen gas. Of the four options, only Mn is one of them.
The acid must be very dilute — in that condition its oxidising power is weak enough for the hydrogen to escape as H₂ before it can be oxidised.
⚗️ The Reaction
Mn(s) + 2HNO₃(aq, very dilute) → Mn(NO₃)₂(aq) + H₂(g)
• Manganese displaces hydrogen from the acid because it lies above hydrogen in the reactivity series.
• Balance check: 1 Mn, 2 H, 2 N and 6 O on each side.
• With concentrated or moderately dilute HNO₃ the same metal would give nitrogen oxides and water instead of H₂.
❌ Why the other options are incorrect
✗ Fe — iron reacts with dilute HCl and dilute H₂SO₄ to give H₂, but with nitric acid the liberated hydrogen is oxidised to water; concentrated HNO₃ even makes iron passive by forming a protective oxide layer.
✗ Zn — zinc gives H₂ readily with dilute HCl/H₂SO₄ (Zn + 2HCl → ZnCl₂ + H₂), but with HNO₃ it yields nitrogen oxides, not hydrogen.
✗ Al — aluminium too gives H₂ with dilute HCl and NaOH, but concentrated HNO₃ renders aluminium passive, forming a tight Al₂O₃ coat that stops further reaction.
📌 SSC & Railway Exam Facts
• Only Mg and Mn liberate H₂ with very dilute HNO₃ — a repeatedly asked NCERT line.
• HNO₃ is an oxidising acid; with other metals it gives N₂O, NO or NO₂, not H₂.
• Metal + dilute acid → salt + hydrogen gas; the gas burns with a pop sound.
• Metals below hydrogen (Cu, Hg, Ag, Au) do not displace hydrogen from dilute acids at all.
• Aqua regia = 3 parts conc. HCl + 1 part conc. HNO₃, the only reagent that dissolves gold and platinum.

Correct Answer: Option 4 — deposition
Deposition (also called desublimation) is the change of a substance directly from the gaseous state to the solid state, with no liquid stage in between. It is the exact reverse of sublimation, in which a solid turns straight into vapour.
It takes place when vapour is cooled so rapidly that its particles lose kinetic energy and are locked into a fixed lattice before they can form a liquid. Deposition is an exothermic change — the vapour must release heat to settle down as a solid.
Familiar examples: the formation of frost on a cold winter morning (water vapour → ice directly), the snowflakes that form in clouds, and the iodine vapour that re-forms shining crystals on the cool upper wall of a vessel after it has sublimed.
❌ Why the other options are incorrect
✗ vaporisation — the change of a liquid into a gas (by boiling or evaporation); it is the wrong direction entirely and involves the liquid state.
✗ solidification — also called freezing, it is the change of a liquid into a solid, e.g. water turning into ice at 0 °C; it starts from a liquid, not a gas.
✗ fusion — another name for melting, the change of a solid into a liquid on absorbing the latent heat of fusion; it is the opposite of freezing.
📌 SSC & Railway Exam Facts
• Sublimation — solid → gas directly; deposition — gas → solid directly.
• Substances that sublime: camphor, naphthalene, iodine, ammonium chloride, dry ice (solid CO₂).
• Melting/fusion — solid → liquid; freezing/solidification — liquid → solid; condensation — gas → liquid.
• Latent heat of fusion of ice = 3.34 × 10⁵ J/kg; latent heat of vaporisation of water = 22.5 × 10⁵ J/kg.
• Frost is a natural example of deposition; dew is a natural example of condensation.

Correct Answer: Option 1 — White
When a magnesium ribbon is heated in air it burns with a dazzling white flame, giving a white powdery ash of magnesium oxide (MgO). This is the very first experiment of the NCERT chapter on chemical reactions, and the phrase “burns with a dazzling white flame” is quoted exactly in exams.
The ribbon must be cleaned with sandpaper before burning, because magnesium reacts slowly with moist air and gets covered by a protective layer of magnesium oxide that would otherwise stop it from catching fire.
The reaction is a combination reaction (two substances form one product) and also an oxidation, since magnesium gains oxygen. The white ash MgO is a basic oxide; dissolved in water it gives magnesium hydroxide, Mg(OH)₂, the milk of magnesia used as an antacid. Because the flame is so bright, magnesium is used in fireworks, flares and old photographic flash bulbs.
⚗️ The Reaction
2Mg(s) + O₂(g) → (burning) 2MgO(s)
• Balance check: 2 Mg and 2 O on each side — the coefficient 2 is needed before both Mg and MgO.
• Magnesium gains oxygen, so it is oxidised; the reaction is both a combination and a redox reaction.
• MgO + H₂O → Mg(OH)₂ — the white ash gives a base, which turns red litmus blue.
❌ Why the other options are incorrect
✗ Red — a crimson-red flame is the flame test for strontium (and lithium), not for magnesium.
✗ Pink — a pink/lilac flame is characteristic of potassium salts in the flame test.
✗ Green — a green flame is given by copper (bluish-green) and barium (apple-green) salts.
📌 SSC & Railway Exam Facts
• Magnesium burns with a dazzling white flame to give white MgO: 2Mg + O₂ → 2MgO.
• Flame colours: Na — golden yellow, K — lilac/pink, Ca — brick red, Cu — bluish green, Ba — apple green.
• MgO + H₂O → Mg(OH)₂ (milk of magnesia), used as an antacid and a mild laxative.
• The magnesium ribbon is cleaned with sandpaper to remove the oxide layer before burning.
• Metal + oxygen → basic oxide; non-metal + oxygen → acidic oxide (e.g. SO₂, CO₂).

Correct Answer: Option 3 — Magnesium
A metal's reaction with water follows its position in the reactivity series. Potassium, sodium and calcium are so reactive that they attack cold water, but magnesium does not react with cold water — it reacts only with hot water, and more vigorously with steam.
Magnesium is placed just below calcium in the series. In cold water the reaction is far too slow to be seen; when the metal is put in hot water, however, hydrogen bubbles stick to its surface and make the ribbon float, giving magnesium hydroxide, Mg(OH)₂. With steam it forms magnesium oxide, MgO, and hydrogen.
Remember the ladder for reaction with water: K, Na, Ca → cold water; Mg → hot water; Al, Zn, Fe → steam only; Pb, Cu, Ag, Au → no reaction with water at all.
⚗️ The Reaction
Mg(s) + 2H₂O(l) → (hot water) Mg(OH)₂(aq) + H₂(g)
Mg(s) + H₂O(g) → (steam) MgO(s) + H₂(g)
• With cold water there is no observable reaction; hot water is needed to start it.
• Balance check for the first equation: 1 Mg, 4 H and 2 O on each side.
• The hydrogen bubbles cling to the ribbon and make it float — the classic laboratory observation.
❌ Why the other options are incorrect
✗ Sodium — Na reacts violently with cold water, and the reaction is so exothermic that the hydrogen catches fire: 2Na + 2H₂O → 2NaOH + H₂ + heat.
✗ Calcium — Ca reacts with cold water too, though less violently; the hydrogen bubbles stick to the metal and make it float: Ca + 2H₂O → Ca(OH)₂ + H₂.
✗ Potassium — K is the most reactive of the four and reacts with cold water most vigorously of all, the hydrogen catching fire at once.
📌 SSC & Railway Exam Facts
• Reaction with water: K, Na, Ca → cold water; Mg → hot water; Al, Zn, Fe → steam; Cu, Ag, Au → none.
• Na and K are stored in kerosene because they catch fire on contact with moist air.
• 3Fe + 4H₂O → (steam) Fe₃O₄ + 4H₂ — iron reacts only with steam, never with cold water.
• Metal + water → metal hydroxide (or oxide) + hydrogen; the H₂ burns with a pop sound.
• Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.

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.

Correct Answer: Option 1 — Heart
The heart is a muscular pumping organ roughly the size of our fist, made of cardiac muscle. It has four chambers — two upper atria and two lower ventricles — and the partition between them prevents oxygenated and deoxygenated blood from mixing.
Deoxygenated blood from the body enters the right atrium, passes to the right ventricle and is pumped to the lungs. Oxygenated blood returns to the left atrium, moves into the left ventricle, and is pumped out through the aorta to the whole body. Because blood passes through the heart twice in one cycle, humans have double circulation.
❌ Why the other options are incorrect
✗ Stomach — a digestive organ; it churns food and secretes HCl and pepsin, it does not pump blood.
✗ Liver — the largest gland of the body; it secretes bile and detoxifies substances.
✗ Pancreas — a mixed gland; it secretes pancreatic juice and the hormones insulin and glucagon.
📌 SSC & Railway Exam Facts
• The left ventricle has the thickest wall because it pumps blood to the whole body.
• Normal human heartbeat is about 72 times per minute; blood pressure is about 120/80 mm Hg.
• William Harvey discovered the circulation of blood.
• Valves in the heart ensure one-way flow of blood.
• The study of the heart is cardiology; the heart is covered by the pericardium.

Correct Answer: Option 1 — Blood
Blood is a fluid connective tissue. Like every connective tissue its cells are loosely spaced and lie in an intercellular matrix; in blood this matrix is liquid and is called plasma.
Floating in the plasma are red blood cells (RBCs) that carry oxygen with the help of haemoglobin, white blood cells (WBCs) that fight infection, and platelets that help in blood clotting. Blood therefore transports gases, digested food, hormones and nitrogenous wastes from one part of the body to another, which is exactly the linking-and-supporting job of a connective tissue.
❌ Why the other options are incorrect
✗ Columnar epithelium — an epithelial tissue of tall pillar-like cells lining the stomach and intestine, meant for absorption and secretion.
✗ Cuboidal epithelium — an epithelial tissue of cube-shaped cells forming the lining of kidney tubules and salivary gland ducts.
✗ Muscle — a separate tissue type, muscular tissue, made of contractile fibres that bring about movement.
📌 SSC & Railway Exam Facts
• The four animal tissues are epithelial, connective, muscular and nervous.
• Plasma forms about 55% of blood; cells form about 45%.
• Bone, cartilage, blood, areolar, adipose, tendon and ligament are all connective tissues.
• Tendon joins muscle to bone; ligament joins bone to bone.
• RBCs have a life span of about 120 days and are made in the bone marrow.

Correct Answer: Option 3 — Helps the cells to grow longer
Auxin is a plant hormone (phytohormone) synthesised at the tip of the shoot. It promotes cell elongation — the cells behind the growing tip take in more water and grow longer, so the shoot gets taller.
This single property explains phototropism. When light falls on one side of a shoot, auxin diffuses towards the shady side. The cells on the shady side elongate more than those on the lighted side, and the unequal growth makes the shoot bend towards the light. Auxin also causes apical dominance and is used to make rooting hormones and weedicides.
❌ Why the other options are incorrect
✗ Make plants insect resistant — insect resistance comes from alkaloids, resins and Bt genes, not from auxin.
✗ Induce flowering — flowering is controlled mainly by florigen and gibberellins along with photoperiod, not by auxin.
✗ Increases stem width — increase in girth is secondary growth caused by the cambium, not by auxin.
📌 SSC & Railway Exam Facts
• Auxin was studied by Charles Darwin and named by F. W. Went.
• Gibberellin causes stem elongation; cytokinin promotes cell division.
• Abscisic acid (ABA) is the stress / growth-inhibitor hormone that causes wilting and closes stomata.
• Ethylene is the only gaseous plant hormone; it ripens fruits.
• 2,4-D is a synthetic auxin used as a weedicide.

Correct Answer: Option 1 — plant cells
Vacuoles are storage sacs for solid or liquid contents, bounded by a single membrane called the tonoplast. In plant cells a single large central vacuole may occupy up to 50–90% of the cell volume, while animal cells have only small vacuoles.
The plant vacuole is filled with cell sap containing water, sugars, amino acids, salts and pigments. Being full of sap it provides turgidity and rigidity to the cell, keeping the plant firm. It also stores waste products and amino acids, sugars and organic acids, keeping them away from the cytoplasm.
❌ Why the other options are incorrect
✗ Bacteria — prokaryotic cells; they lack membrane-bound organelles, so no true large vacuole (only gas vacuoles in some).
✗ Animal cells — they do have vacuoles, but these are small and numerous, never a single large central one.
✗ Both plant cells and animal cells — large vacuoles are characteristic of plant cells only, so this is wrong.
📌 SSC & Railway Exam Facts
• The membrane around the vacuole is the tonoplast.
• In Amoeba the food vacuole digests food and the contractile vacuole removes excess water.
• Plastids, cell wall and large central vacuole are present in plant cells but absent in animal cells.
• Centrosome and lysosomes in large numbers are typical of animal cells.
• Lysosome is the suicidal bag; mitochondrion is the powerhouse of the cell.

Correct Answer: Option 4 — Amoeba
A parasite is an organism that lives on or inside another living organism, the host, and derives its food from it while harming the host. This mode of heterotrophic nutrition is called parasitic nutrition.
Amoeba is not a parasite. It is a free-living unicellular protozoan of pond water that shows holozoic nutrition: it engulfs solid food particles by forming pseudopodia (false feet) in a process called phagocytosis, digests them in a food vacuole, absorbs the useful part and throws out the undigested matter anywhere on its surface.
❌ Why the other options are incorrect
✗ Ticks — ectoparasites that cling to the skin of cattle, dogs and humans and suck blood.
✗ Lice — ectoparasites of the human scalp and body that also live by sucking blood.
✗ Cuscuta (Amarbel) — a parasitic plant with no chlorophyll; it twines around a host and sucks sap through haustoria.
📌 SSC & Railway Exam Facts
• Modes of heterotrophic nutrition: holozoic, saprophytic and parasitic.
• Amoeba, Paramecium and human beings show holozoic nutrition.
• Fungi, bread mould (Rhizopus), mushroom and yeast are saprophytes.
• Entamoeba histolytica causes amoebic dysentery; Plasmodium causes malaria.
• Rafflesia is a root parasite; Mistletoe (Viscum) is a partial stem parasite.

Correct Answer: Option 2 — Hydra
Hydra is a simple multicellular animal that reproduces asexually by budding. It uses its regenerative cells to make the bud: these cells divide repeatedly at one point of the body and form a small outgrowth called a bud.
The bud develops its own mouth and tentacles while still attached to the parent, and when it is mature it detaches and lives as a new independent Hydra. Because the offspring comes from a single parent by mitosis, it is genetically identical to the parent — a clone.
❌ Why the other options are incorrect
✗ Spirogyra — a filamentous alga that reproduces by simple fragmentation; the filament breaks into pieces that grow into new individuals.
✗ Planaria — a flatworm famous for regeneration; a cut piece grows into a whole worm, but this is not budding.
✗ Amoeba — reproduces by binary fission, splitting into two daughter cells, not by budding.
📌 SSC & Railway Exam Facts
• Yeast also reproduces by budding, but it is unicellular.
• Binary fission — Amoeba, Leishmania, Plasmodium (multiple fission).
• Fragmentation — Spirogyra; Regeneration — Hydra and Planaria.
• Spore formation — Rhizopus (bread mould); Vegetative propagation — potato, Bryophyllum.
• Hydra belongs to the phylum Coelenterata (Cnidaria).

Correct Answer: Option 3 — Urethra - a pair
The human excretory system consists of a pair of kidneys, a pair of ureters, one urinary bladder and one urethra. So the urethra is single (unpaired), and calling it "a pair" is the incorrect match.
The bean-shaped kidneys filter the blood through about one million nephrons each, forming urine. The two ureters carry this urine down to the urinary bladder, where it is stored. From the bladder, urine is finally passed out of the body through the single urethra, whose opening is guarded by a sphincter muscle that gives voluntary control over urination.
❌ Why the other options are incorrect
✗ Ureter - a pair — correct; one ureter arises from each kidney and opens into the bladder.
✗ Kidney - a pair — correct; two bean-shaped kidneys lie on either side of the backbone in the abdomen.
✗ Urinary bladder - unpaired — correct; there is only one muscular bladder storing urine.
📌 SSC & Railway Exam Facts
• The nephron is the structural and functional unit of the kidney.
• Filtration happens in the Bowman's capsule + glomerulus (Malpighian body).
• An adult produces about 1–1.8 litres of urine a day; urine is about 95% water.
• The main nitrogenous waste in human urine is urea, made in the liver.
• Haemodialysis is the artificial kidney treatment for kidney failure.

Correct Answer: Option 3 — Nutrition
Nutrition is defined as the process by which an organism takes in food from outside its body, digests it and uses it to obtain energy and materials for growth, repair and maintenance of the body. The food is the organism's source of energy, so its transfer from outside to inside is exactly nutrition.
Nutrition is of two broad kinds. Autotrophic nutrition — green plants prepare their own food by photosynthesis. Heterotrophic nutrition — animals and fungi take ready-made food; it is further of three types, holozoic (Amoeba, humans), saprophytic (fungi) and parasitic (Cuscuta, ticks).
❌ Why the other options are incorrect
✗ Transportation — carrying of substances within the body (blood carrying oxygen and food), not intake from outside.
✗ Translocation — movement of prepared food from leaves to other parts of a plant through the phloem.
✗ Metabolism — the sum of all chemical reactions (anabolism + catabolism) already going on inside the body.
📌 SSC & Railway Exam Facts
• Steps of nutrition: ingestion, digestion, absorption, assimilation and egestion.
• Amoeba shows holozoic nutrition using pseudopodia.
• The five main nutrients are carbohydrates, proteins, fats, vitamins and minerals.
• Photosynthesis occurs in chloroplasts and needs CO₂, water, sunlight and chlorophyll.
• Digestion of starch begins in the mouth by the enzyme salivary amylase (ptyalin).

Correct Answer: Option 1 — Lactic acid
During sudden vigorous exercise the oxygen supply to the muscles falls short of the demand. The muscle cells then switch from aerobic respiration to anaerobic respiration, in which pyruvate is converted into lactic acid and only a little energy is released.
This lactic acid accumulates in the muscle fibres and irritates them, producing the painful cramps we feel. When we rest and breathe deeply, the extra oxygen taken in — the oxygen debt — oxidises the lactic acid, and the cramp is relieved.
❌ Why the other options are incorrect
✗ Glucose — the starting fuel of respiration, broken down in the cytoplasm; its accumulation does not cause cramps.
✗ Sucrose — a disaccharide (table sugar) of glucose + fructose; it is a transport sugar in plants, not a muscle waste.
✗ Pyruvic acid — the 3-carbon intermediate formed in glycolysis; it is quickly converted to lactic acid, so it is not what builds up.
📌 SSC & Railway Exam Facts
• Glycolysis occurs in the cytoplasm; aerobic breakdown of pyruvate occurs in the mitochondria.
• In yeast, anaerobic respiration gives ethanol + CO₂ — this is fermentation.
• Aerobic respiration yields 38 ATP per glucose; anaerobic gives only 2 ATP.
• The energy currency of the cell is ATP (adenosine triphosphate).
• Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

Correct Answer: Option 4 — Muscular tissue
Muscular tissue consists of long cells called muscle fibres, which contain contractile proteins (actin and myosin). These proteins contract and relax, and this shortening and lengthening of the fibres brings about every movement of the body.
There are three types. Striated (skeletal / voluntary) muscles are attached to bones and move the limbs under our will. Smooth (unstriated / involuntary) muscles line the alimentary canal, blood vessels and urinary bladder. Cardiac muscle is found only in the heart, and it contracts rhythmically and involuntarily throughout life.
❌ Why the other options are incorrect
✗ Cartilage — a connective tissue that gives flexible support to the nose, ear, larynx and trachea and smoothens bone surfaces at joints.
✗ Epithelial tissue — the covering and lining tissue of body surfaces; it protects, absorbs and secretes, but cannot contract.
✗ Areolar tissue — a loose connective tissue found between skin and muscles that fills spaces and repairs tissues.
📌 SSC & Railway Exam Facts
• Striated muscles are voluntary and multinucleate; smooth muscles are involuntary and spindle-shaped.
• Cardiac muscle is striated but involuntary and never fatigues.
• Tendons attach skeletal muscles to bones.
• Actin and myosin are the contractile proteins of muscle.
• The biceps is a flexor and the triceps an extensor of the arm.

Correct Answer: Option 4 — Undigested food
Plasma is the liquid matrix of blood — a pale yellow fluid that makes up about 55% of the blood volume and is about 90–92% water. Dissolved in it are plasma proteins (albumin, globulin, fibrinogen), hormones, mineral salts, digested food such as glucose and amino acids, and wastes such as urea and carbon dioxide.
Undigested food never enters the blood at all. Only food that has been completely digested into small soluble molecules can be absorbed through the villi of the small intestine into the blood. Undigested material passes on into the large intestine and is thrown out of the body as faeces (egestion).
❌ Why the other options are incorrect
✗ Proteins — plasma proteins like fibrinogen (clotting), albumin (osmotic balance) and globulin (immunity) are always present.
✗ Hormones — endocrine glands pour hormones directly into the blood, which carries them to their target organs.
✗ Salts — sodium, potassium, calcium and bicarbonate ions are dissolved in plasma and maintain its pH and balance.
📌 SSC & Railway Exam Facts
• Blood = plasma (55%) + formed elements (45%); serum is plasma without fibrinogen.
• Villi in the small intestine absorb digested food into the blood.
• Plasma carries CO₂ mostly as bicarbonate and urea to the kidneys.
• Fibrinogen + platelets bring about blood clotting.
• The pH of human blood is about 7.4, slightly alkaline.

Correct Answer: Option 3 — Kulhad – non-biodegradable
A kulhad is a cup made of baked clay (earthen ware). Since it is made of natural soil, a broken kulhad simply mixes back into the earth and is treated as an eco-friendly, biodegradable item. Calling it non-biodegradable is therefore the incorrect pair.
Biodegradable wastes are substances that are broken down into simple harmless matter by micro-organisms such as bacteria and fungi — vegetable peels, paper, cow dung, cotton and wood. Non-biodegradable wastes cannot be decomposed by microbes and stay in the environment for hundreds of years — plastics, polythene, glass, metals and DDT.
❌ Why the other options are incorrect
✗ Plastic glass – non-biodegradable — correct pairing; plastic is a polymer that microbes cannot decompose.
✗ Vegetable – biodegradable — correct pairing; vegetable matter is organic and rots quickly by microbial action.
✗ Paper cup – biodegradable — correct pairing; paper is made of cellulose from plants and is decomposed by microbes.
📌 SSC & Railway Exam Facts
• Green dustbins are for biodegradable waste and blue dustbins for non-biodegradable waste.
• Vermicomposting uses earthworms to convert organic waste into manure.
• Non-biodegradable pesticides like DDT cause biomagnification along the food chain.
• The 3 Rs of waste management are Reduce, Reuse and Recycle.
• Biogas (gobar gas) is mainly methane (CH₄) produced from cattle dung.

Correct Answer: Option 3 — Seed
Vegetative propagation is a type of asexual reproduction in which a new plant grows from a vegetative part of the parent plant — the root, stem, leaf or bud. No gametes and no fertilisation are involved, so the new plants are clones of the parent.
A seed, on the other hand, is the product of sexual reproduction. It is formed inside the fruit after fertilisation of the egg by the male gamete from the pollen grain, and it carries characters of two parents. Because it is a reproductive (not vegetative) structure, growing a plant from a seed is not vegetative propagation.
❌ Why the other options are incorrect
✗ Leaves — Bryophyllum grows new plants from buds in the notches of its leaf margin.
✗ Stem — potato (tuber), ginger, sugarcane, rose and grape are all propagated by stem pieces or cuttings.
✗ Root — sweet potato, dahlia and Asparagus grow new shoots from their roots.
📌 SSC & Railway Exam Facts
• Artificial methods of vegetative propagation: cutting, layering, grafting and tissue culture.
• Vegetative propagation keeps the desired characters unchanged and gives earlier flowering and fruiting.
• Banana, orange, rose and jasmine are grown this way as they have lost the capacity to make viable seeds.
• Tissue culture grows whole plants from a few cells on a nutrient medium (micropropagation).
• The eye of a potato is a bud that sprouts into a new plant.

Correct Answer: Option 2 — F × s
📖 Given
• Constant force applied, F
• Distance moved in the direction of the force, s
• The force and the displacement are in the same direction (θ = 0°)
✍️ Formula
W = F s cos θ
🔢 Solution
• Here the trolley moves in the direction of the push, so θ = 0°
• cos 0° = 1
• W = F × s × 1
• W = F × s
So the work done is simply the product of the force and the distance moved along it — if the girl pushes with 20 N through 5 m, she does 20 × 5 = 100 J of work.
❌ Why the other options are incorrect
✗ F + s — force and distance are different physical quantities with different units (N and m); they can never be added.
✗ F/s — dividing force by distance gives N/m, which is the unit of a spring constant, not of work.
✗ F − s — subtracting two unlike quantities is meaningless; the result would not even have the unit of work.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit joule (J), and 1 J = 1 N × 1 m.
• Work is a scalar quantity, although force and displacement are both vectors.
• θ = 0° → positive work, θ = 180° → negative work, θ = 90° → zero work.
• CGS unit of work is the erg; 1 J = 10⁷ erg.
• Power = W/t, SI unit watt (W); 1 horsepower = 746 W.

Correct Answer: Option 1 — kinetic energy
The energy possessed by a body because of its motion is called kinetic energy (KE). A moving body can do work on anything that comes in its way — a moving hammer drives a nail, flowing water turns a turbine, and a moving bullet pierces a target.
Its value is given by KE = ½mv², where m is the mass and v the velocity of the body. Since KE depends on the square of the velocity, doubling the speed makes the kinetic energy four times larger, while doubling the mass only doubles it.
Kinetic energy is a scalar quantity, it is always positive or zero, and it is measured in joules (J).
❌ Why the other options are incorrect
✗ potential energy — this is the energy of position or configuration, PE = mgh, as in water stored in a dam or a stretched spring; it does not need motion.
✗ electromagnetic energy — the energy carried by electromagnetic waves such as light, radio waves and X-rays, not the energy of a moving body.
✗ nuclear energy — the energy released from the nucleus of an atom during fission or fusion, as in a nuclear reactor or the sun.
📌 SSC & Railway Exam Facts
• KE = ½mv² and p = mv, so KE = p²/2m.
• Work–energy theorem: work done by the net force = change in kinetic energy.
• Mechanical energy = KE + PE, and it stays constant when only gravity acts.
• SI unit of energy is the joule (J); 1 calorie = 4.2 J; 1 kWh = 3.6 × 10⁶ J.
• A body at rest has zero kinetic energy but may still possess potential energy.

Correct Answer: Option 2 — less than the weight on the earth
📖 Given
• Weight is the gravitational pull on a body: W = mg
• Mass of the moon is about 1/100 of the earth's mass
• Radius of the moon is about 1/4 of the earth's radius
✍️ Formula
g = GM/R² and W = mg
🔢 Solution
• g(moon)/g(earth) = (M(moon)/M(earth)) × (R(earth)/R(moon))²
• = (1/100) × (4)² = 16/100
• So g(moon) ≈ 1/6 of g(earth), about 1.6 m/s²
• Since mass m stays the same, W(moon) = m × g(moon) = 1/6 of W(earth)
So a 60 kg astronaut weighing 600 N on the earth weighs only about 100 N on the moon — which is why astronauts can take those long, floating leaps.
❌ Why the other options are incorrect
✗ greater than the weight on the earth — impossible, because the moon's gravitational acceleration is much smaller than the earth's.
✗ equal to the weight on the earth — only the mass stays the same everywhere; weight changes with the value of g.
✗ may be greater than or less than depending on the density of the object — the density of the object plays no part; the ratio g(moon)/g(earth) = 1/6 is the same for every body.
📌 SSC & Railway Exam Facts
• Weight on the moon = 1/6 of the weight on the earth; g(moon) ≈ 1.6 m/s².
• Mass is constant and measured in kg; weight = mg is a force, measured in newton (N).
• g is maximum at the poles, minimum at the equator, and zero at the centre of the earth.
• Weight becomes zero in a freely falling lift — the state of weightlessness.
• The moon has no atmosphere because its escape velocity (2.4 km/s) is too small to hold gases.

Correct Answer: Option 4 — Between the centre of curvature and the focus point
For a concave mirror, the position of the image depends entirely on where the object is placed with respect to the pole P, the focus F and the centre of curvature C.
When the object is placed beyond C, the reflected rays meet at a point lying between F and C. The image so formed is real, inverted and diminished (smaller than the object).
This can be checked with the mirror formula 1/v + 1/u = 1/f. Taking f = −10 cm (so C is at −20 cm) and an object at u = −30 cm, we get v = −15 cm — a point lying between F (−10 cm) and C (−20 cm), exactly as stated.
❌ Why the other options are incorrect
✗ At the centre of curvature — the image forms at C only when the object itself is placed at C; then the image is real, inverted and of the same size.
✗ At infinity — the image goes to infinity only when the object is placed at the focus F; the reflected rays then emerge parallel.
✗ At the focus — the image forms at F only when the object is at infinity; the image is then highly diminished and point-sized.
📌 SSC & Railway Exam Facts
• Concave mirror ray table: object at ∞ → image at F; beyond C → between F and C; at C → at C; between C and F → beyond C; at F → at ∞; between P and F → behind the mirror, virtual and enlarged.
• Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u; R = 2f.
• A concave mirror forms a virtual, erect, enlarged image only when the object lies between P and F.
• Concave mirrors are used in torches, headlights, solar cookers, shaving mirrors and dentists' mirrors.
• By the sign convention, distances measured against the incident light are negative.

Correct Answer: Option 1 — Magnetic field lines are not closed curves.
This statement is false. Magnetic field lines are always closed, continuous curves. Outside a magnet they run from the north pole to the south pole, and inside the magnet they continue from the south pole back to the north pole, closing the loop.
This happens because isolated magnetic poles (monopoles) do not exist — a magnet always has both poles together, so the lines can never simply begin or end somewhere in space. Electric field lines, by contrast, do start on positive charges and end on negative charges, and are therefore not closed.
Since the question asks which statement is NOT true, the answer is this first option.
❌ Why the other options are incorrect
✗ Inside the magnet, the direction of field lines is from its south pole to its north pole. — this is true; it is exactly what makes the lines closed loops.
✗ No two field lines are found to cross each other. — true; if they crossed, the compass needle would show two directions of the field at one point, which is impossible.
✗ The relative strength of the magnetic field is shown by the degree of closeness of the field lines. — true; crowded lines mean a stronger field, and the lines are closest near the poles.
📌 SSC & Railway Exam Facts
• Magnetic field lines are closed curves: N → S outside, S → N inside the magnet.
• SI unit of magnetic field is the tesla (T); the CGS unit is the gauss; 1 T = 10⁴ gauss.
• Field lines are drawn with a compass needle or with iron filings sprinkled around a magnet.
• Magnetic monopoles do not exist — breaking a magnet gives two smaller magnets, each with both poles.
• Oersted (1820) showed that a current-carrying wire deflects a compass needle.

Correct Answer: Option 1 — 10 N
📖 Given
• Mass of the metal box, m = 1 kg
• Acceleration due to gravity, g = 10 m/sec²
• The box rests on a horizontal table surface
✍️ Formula
Thrust = weight of the box = mg (for a body resting on a horizontal surface)
🔢 Solution
• Thrust = m × g
• Thrust = 1 × 10
• Thrust = 10 N
Thrust is the force acting perpendicular (normal) to a surface. Here the box presses down on the table with its full weight, so the thrust on the table is exactly 10 newtons.
❌ Why the other options are incorrect
✗ 100 N — this comes from wrongly taking the mass as 10 kg, or from multiplying by 10 one extra time.
✗ 1 N — this is just the numerical value of the mass; the mass has not been multiplied by g.
✗ 1000 N — this would need a mass of 100 kg; it is 100 times the correct answer.
📌 SSC & Railway Exam Facts
• Thrust is the force acting normal to a surface; SI unit newton (N).
• Pressure = thrust/area, P = F/A; SI unit pascal (Pa), and 1 Pa = 1 N/m².
• The same thrust on a smaller area gives a larger pressure — that is why a sharp knife cuts better.
• Atmospheric pressure at sea level ≈ 1.013 × 10⁵ Pa = 1 atm, measured by a barometer.
• Archimedes' principle: the upthrust on a body in a fluid equals the weight of the fluid displaced.

Correct Answer: Option 4 — kg m s⁻¹
📖 Given
• Mass of the body, m, measured in kilogram (kg)
• Velocity of the body, v, measured in metre per second (m s⁻¹)
✍️ Formula
Momentum, p = m × v
🔢 Solution
• Unit of p = (unit of mass) × (unit of velocity)
• Unit of p = kg × m s⁻¹
• Unit of p = kg m s⁻¹ (also written as kg m/s)
Momentum measures the quantity of motion in a body. A heavy truck moving slowly and a light bullet moving fast can both have a large momentum, which is why both are hard to stop.
❌ Why the other options are incorrect
✗ kg m — the second is missing altogether; mass × distance is not momentum.
✗ kg m⁻¹ s⁻¹ — here metre carries a negative power; momentum has metre in the numerator, since velocity is m/s.
✗ kg m s — this multiplies by time instead of dividing by it, which is the exact opposite of m/s.
📌 SSC & Railway Exam Facts
• p = mv; momentum is a vector quantity and points along the velocity.
• Newton's second law: F = ma = dp/dt — force is the rate of change of momentum.
• Impulse = F × t = change in momentum, measured in N s, and 1 N s = 1 kg m s⁻¹.
• Law of conservation of momentum: in the absence of an external force, total momentum stays constant.
• Recoil of a gun, rocket propulsion and collisions are all explained by conservation of momentum.

Correct Answer: Option 4 — Convex mirror
A convex mirror is used as the rear-view (wing) mirror in vehicles. Its reflecting surface bulges outwards, so it always forms an image that is virtual, erect and diminished, formed between the pole P and the focus F behind the mirror.
Two properties make it ideal for this job:
• It gives an erect image, so the driver sees the traffic behind the right way up.
• It has a much wider field of view than a plane mirror of the same size, because the diminished image lets a large region behind fit into a small mirror.
The only drawback is that objects look smaller and therefore farther away than they really are — which is why such mirrors carry the warning "objects in the mirror are closer than they appear".
❌ Why the other options are incorrect
✗ Plano-convex lens — a lens refracts light and is used in optical instruments; a rear-view mirror must reflect light, not transmit it.
✗ Biconvex lens — again a converging lens, used in magnifying glasses, cameras and to correct hypermetropia, not as a mirror.
✗ Concave mirror — for distant objects it gives a real, inverted image and has a narrow field of view, so it would show the traffic upside down.
📌 SSC & Railway Exam Facts
• Convex mirror: image is always virtual, erect and diminished; used in rear-view mirrors and shop security mirrors.
• Concave mirror: used in torches, headlights, solar cookers, shaving mirrors and dentists' mirrors.
• Plane mirror gives a virtual, erect, same-sized, laterally inverted image.
• Mirror formula: 1/v + 1/u = 1/f; m = −v/u; f = R/2.
• For a convex mirror, f is positive; for a concave mirror, f is negative.

Correct Answer: Option 2 — 200 J
📖 Given
• Mass of the rice bag, m = 10 kg
• Height to which it is raised, h = 2 m
• Acceleration due to gravity, g = 10 m s⁻²
✍️ Formula
The worker lifts the bag against gravity, so W = F × h = mgh
🔢 Solution
• Force needed = weight of the bag = m × g
• F = 10 × 10 = 100 N
• W = F × h
• W = 100 × 2
• W = 200 J
The 200 joules of work done by the worker is stored in the bag as gravitational potential energy, which it would release if it fell back to the ground.
❌ Why the other options are incorrect
✗ 100 J — this is only mg, the force in newtons; the student stopped before multiplying by the height h = 2 m.
✗ 59 J — no combination of 10 kg, 2 m and g = 10 m s⁻² produces this value; it is a distractor.
✗ 153 J — again unrelated to the data; mgh must come out as a clean multiple of 100 here.
📌 SSC & Railway Exam Facts
• Work done against gravity, W = mgh, which equals the gain in potential energy.
• W = F s cos θ; here the force and the displacement are both vertical, so cos 0° = 1.
• SI unit of work and energy is the joule (J); 1 J = 1 N m.
• Potential energy depends only on the height, not on the path taken to reach it.
• If the worker had merely carried the bag horizontally, the work done against gravity would be zero (θ = 90°).

Correct Answer: Option 3 — +1.75 D
📖 Given
• Power of the first lens, P₁ = +2.0 D (a converging / convex lens)
• Power of the second lens, P₂ = −0.25 D (a diverging / concave lens)
• The two lenses are placed in contact
✍️ Formula
P = P₁ + P₂ — powers of thin lenses in contact simply add, with their signs
🔢 Solution
• P = (+2.0) + (−0.25)
• P = 2.0 − 0.25
• P = +1.75 D
The positive sign shows that the combination still behaves as a converging (convex) lens. Its focal length is f = 1/P = 1/1.75 ≈ 0.57 m, that is about 57 cm.
❌ Why the other options are incorrect
✗ +2.25 D — this is 2.0 + 0.25; the minus sign of the second lens has been ignored while adding.
✗ +1.25 D — this would follow from subtracting 0.75 D, a value that is simply not given in the question.
✗ −3.5 D — this has both the wrong magnitude and the wrong sign; a +2.0 D lens dominates a −0.25 D lens, so the result must be positive.
📌 SSC & Railway Exam Facts
• Power P = 1/f (f in metres); SI unit dioptre (D), and 1 D = 1 m⁻¹.
• For lenses in contact: P = P₁ + P₂ + P₃ … and 1/f = 1/f₁ + 1/f₂ + ….
• Convex lens → positive power; concave lens → negative power.
• Lens formula: 1/v − 1/u = 1/f; magnification m = v/u.
• Myopia is corrected by a concave lens and hypermetropia by a convex lens.

Correct Answer: Option 2 — either positive or negative
The work done by a force is W = F s cos θ, where θ is the angle between the force and the displacement. The sign of the work is decided entirely by cos θ, so a constant force can do work of either sign:
• θ = 0° → cos 0° = +1 → W = +Fs, the work is positive. Example: gravity acting on a falling stone.
• θ = 180° → cos 180° = −1 → W = −Fs, the work is negative. Example: friction on a sliding box, or gravity on a stone thrown upward.
So the very same constant force — gravity — does positive work in one situation and negative work in another. Hence the work done by a constant force is either positive or negative, and cannot be fixed to one sign.
❌ Why the other options are incorrect
✗ always positive — false, because a force acting opposite to the displacement does negative work, as brakes do on a moving train.
✗ always zero — the work is zero only in the special case θ = 90°, as for a coolie carrying a load horizontally on his head.
✗ always negative — false, because a force acting along the displacement does positive work, as when you push a trolley forward.
📌 SSC & Railway Exam Facts
• W = F s cos θ; SI unit joule (J); work is a scalar quantity.
• Positive work adds energy to a body; negative work removes energy from it.
• In uniform circular motion, the centripetal force does zero work (θ = 90°).
• Work–energy theorem: net work done = change in kinetic energy.
• 1 J = 1 N m = 10⁷ erg; 1 calorie = 4.2 J.

Correct Answer: Option 3 — F = G (M × m)/d²
Newton's universal law of gravitation states that every object in the universe attracts every other object with a force that is
• directly proportional to the product of their masses: F ∝ M × m
• inversely proportional to the square of the distance between them: F ∝ 1/d²
Combining the two, F ∝ Mm/d², and introducing the constant of proportionality G,
F = G (M × m)/d²
Here G = 6.67 × 10⁻¹¹ N m² kg⁻² is the universal gravitational constant, so called because its value is the same for every pair of bodies anywhere in the universe. The force acts along the line joining the two bodies and is always attractive.
❌ Why the other options are incorrect
✗ F = G 1/d² — the masses are missing; the force must depend on both M and m, otherwise a heavy and a light body would be attracted equally.
✗ F = G (M × m)/d — here d appears to the first power; gravitation is an inverse square law, so d must be squared.
✗ F = G M/d² — only one mass appears; the product of the two masses is required by the law.
📌 SSC & Railway Exam Facts
• F = GMm/d² — an inverse square law; doubling the distance makes the force one-fourth.
• G = 6.67 × 10⁻¹¹ N m² kg⁻², measured experimentally by Henry Cavendish.
• On the earth's surface, g = GM/R² ≈ 9.8 m/s²; g and G are different quantities.
• The gravitational forces on the two bodies are equal and opposite — an action–reaction pair.
• Gravitation explains the motion of planets, tides, satellites and the free fall of bodies.

Correct Answer: Option 4 — 9 J
📖 Given
• Mass of the ball, m = 0.5 kg
• Uniform velocity, v = 6 m/s
✍️ Formula
KE = ½mv²
🔢 Solution
• KE = ½ × 0.5 × (6)²
• v² = 6 × 6 = 36
• KE = ½ × 0.5 × 36
• KE = 0.25 × 36
• KE = 9 J
So the moving ball carries 9 joules of energy, and it is exactly this much work that must be done against it to bring it to rest.
❌ Why the other options are incorrect
✗ 6 J — this is just the numerical value of the velocity; the mass and the factor ½ have been left out completely.
✗ 15 J — this needs v² = 60, since ½ × 0.5 × 60 = 15; but squaring 6 gives 36, not 60.
✗ 12 J — this needs v² = 48, since ½ × 0.5 × 48 = 12; again the square of 6 is 36, so 12 J cannot arise.
📌 SSC & Railway Exam Facts
• KE = ½mv² — kinetic energy depends on the square of the velocity.
• Doubling the speed makes the kinetic energy four times; doubling the mass only doubles it.
• KE = p²/2m, where p = mv is the momentum.
• Work–energy theorem: net work done on a body = change in its kinetic energy.
• SI unit of kinetic energy is the joule (J); it is a scalar and is never negative.