BPSC 70th CCE Prelims General Studies · Question 50 of 143

Correct — A, ii and iii. An exothermic process gives out heat to its surroundings, so its enthalpy change is negative; an endothermic process takes heat in. Sort the four accordingly. Statement (ii), diluting concentrated sulphuric acid, releases a great deal of heat as the acid molecules become hydrated — enough to make the mixture spit and, if done carelessly, to boil the water at the surface. That is the whole reason for the laboratory rule that acid is poured into water in a thin stream with constant stirring, never water into acid: a large volume of water, with its high specific heat of about 4.18 kilojoules per kilogram per kelvin, can soak up the heat, whereas a splash of water landing on concentrated acid cannot. Statement (iii) is the slaking of quicklime, CaO + H₂O → Ca(OH)₂, the standard school example of a combination reaction that is strongly exothermic; the mixture heats up sharply and can steam. Both release heat, so both are exothermic. Statements (i) and (iv) are not reactions at all but phase changes in the same direction — liquid to vapour for water, solid straight to vapour for camphor — and both require latent heat to be supplied to break the molecules free of one another. Evaporation is why sweat cools the skin and why water stays cool in an earthen pot; sublimation is why a camphor tablet left in the open shrinks away without ever melting into a puddle. Both absorb heat and are therefore endothermic. So the exothermic pair is ii and iii, which is option (a).

• (b)iii and iv — Correctly identifies the slaking of quicklime but pairs it with the sublimation of camphor. Sublimation converts a solid directly to vapour and must absorb the enthalpy of sublimation to do it, so it cools its surroundings rather than warming them — the opposite of what this option claims.
• (c)i and ii — Gets the dilution of sulphuric acid right and then adds the evaporation of water. Evaporation is the classic endothermic phase change; it is the reason a wet cloth on the forehead cools a patient and the reason desert coolers work. Heat flows into the water, not out of it.
• (d)i and iv — The exact inverse of the answer: it selects the only two processes that are unambiguously endothermic and omits both of the ones that release heat. It is what a candidate marks after registering that evaporation and sublimation are the two 'physical' items and assuming the question wanted the pair that behaves alike.

Every physical or chemical change involves an exchange of heat with its surroundings, and the sign of that exchange is what 'exothermic' and 'endothermic' describe. Exothermic means heat flows out of the system — combustion, respiration, neutralisation, the slaking of lime, and the hydration that occurs when concentrated acids are diluted. Endothermic means heat flows in — photosynthesis, the thermal decomposition of calcium carbonate in a lime kiln, and all phase changes that move matter to a less condensed state. That last group is the reliable rule. Melting, vaporisation and sublimation each pull molecules further apart against the forces holding them together, so each requires latent heat to be supplied; freezing, condensation and deposition run the other way and give the same heat back. Water's latent heat of vaporisation is about 2,260 kilojoules per kilogram, roughly seven times its latent heat of fusion of about 334 kilojoules per kilogram, which is why evaporation is such an effective coolant. Dissolution is the one category that goes either way: acids and alkalis usually dissolve exothermically, while ammonium nitrate and potassium nitrate dissolve endothermically and chill the solution.

There is a shortcut here that removes the chemistry altogether. Two of the four items — evaporation of water and sublimation of camphor — are phase changes to a less condensed state, and both must therefore absorb heat. Any option containing (i) or (iv) is consequently wrong, and options (b), (c) and (d) each contain one of them. Only option (a) survives, and you have reached it without needing to know anything about sulphuric acid or quicklime. That is the discipline worth taking from this question: classify each item by type before you evaluate it, because a single structural rule often disposes of three options at once. If you do want to check the chemistry, both of the surviving items are combination or hydration processes, and bond formation releases energy — new bonds forming in Ca(OH)₂ and new ion–water interactions forming in dilute acid. The one caution is that dissolving is not automatically exothermic; whether a salt warms or cools its solution depends on whether hydration energy exceeds lattice energy, which is why instant cold packs work at all.

• Exothermic means heat is released and the enthalpy change is negative; endothermic means heat is absorbed and the enthalpy change is positive
• All phase changes towards a less condensed state — melting, evaporation, boiling and sublimation — absorb latent heat; freezing, condensation and deposition release it
• Water's latent heat of vaporisation is about 2,260 kilojoules per kilogram against a latent heat of fusion of about 334 kilojoules per kilogram, and its specific heat is about 4.18 kilojoules per kilogram per kelvin
• Slaking of quicklime, CaO + H₂O → Ca(OH)₂, is a strongly exothermic combination reaction; the slaked lime produced is used for whitewashing, where it slowly reacts with atmospheric carbon dioxide to form calcium carbonate
• Dissolution can go either way — concentrated sulphuric acid releases heat on dilution, while ammonium nitrate and potassium nitrate absorb heat and cool their solution, the principle behind instant cold packs

• Assuming that anything involving water and heat must be exothermic; evaporation and sublimation both take heat in
• Treating all dissolution as exothermic — ammonium nitrate and potassium nitrate cool their solution
• Confusing the temperature of a substance with the heat it contains; steam and boiling water are both at 100 °C, yet steam carries the latent heat of vaporisation in addition

BPSC sets this as a four-statement classification with numbered items and paired options, so the fastest route is a structural rule that kills several options at once rather than four separate judgements. UPSC prefers the same physics in assertion-reason or statement form — why acid is added to water rather than water to acid, or whether steam and boiling water hold the same heat — which tests the mechanism rather than the label.

• practice — not a real PYQ

  Which one of the following processes is endothermic ?

  • (a)Condensation of steam into water
  • (b)Sublimation of iodine crystals
  • (c)Slaking of quicklime with water
  • (d)Neutralisation of hydrochloric acid by sodium hydroxide

  Answer(b) Sublimation of iodine crystals — a phase change to a less condensed state, which absorbs heat; the other three all release heat.

• practice — not a real PYQ

  The product formed when quicklime reacts with water is

  • (a)Calcium carbonate
  • (b)Calcium hydroxide
  • (c)Calcium sulphate
  • (d)Calcium oxide

  Answer(b) Calcium hydroxide — CaO + H₂O gives Ca(OH)₂ with a large release of heat; the calcium carbonate of option (a) forms later, when whitewash absorbs carbon dioxide from the air.