Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : Evaporation of sweat from human skin dissipates body heat.
Reason (R) : High humidity on a

Question

Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : Evaporation of sweat from human skin dissipates body heat.
Reason (R) : High humidity on a hot day increases discomfort.
Select the correct answer from the code given below :
Code :

UnlockIAS · Accepted Answer

Correct — A, both (A) and (R) are true, but (R) is not the correct explanation of (A). Read that option text carefully, because this booklet has inverted the code you have drilled: on this paper (a) is the 'both true, R does NOT explain A' slot and (d) is the 'both true and R IS the explanation' slot — the reverse of the usual UPSC arrangement. Now the substance. The Assertion is true, and it is straightforward physics. Turning liquid sweat into vapour costs energy — the latent heat of vaporisation, roughly 2,400 kilojoules for every litre of sweat that actually evaporates — and that energy is drawn out of the skin and the blood flowing beneath it. The body is therefore cooled. This route matters most exactly when the others fail: radiation, conduction and convection all need the surroundings to be cooler than the skin, so once air temperature climbs towards skin temperature, evaporation is very nearly the only way left to shed heat. The Reason is also true. Humid heat is more uncomfortable than dry heat of the same temperature, which is why weather services report a heat index rather than the bare thermometer reading. But the Reason does not explain the Assertion. It is the other way round. Because cooling depends on evaporation, and because evaporation slows when the surrounding air is already close to saturated — the vapour-pressure difference between wet skin and humid air collapses — sweat then drips off instead of evaporating, less heat leaves the body, and the person feels worse. The Assertion is the cause and the Reason is the consequence; the explanatory arrow runs A to R, not R to A. Both true, R does not explain A, and on this paper that is option (a).

Why the other options are wrong — (b) This is the single most dangerous option on the page, and not because the physics in it is tempting. It is what a candidate marks who reasons about the substance perfectly — both statements true, R does not explain A — and then reaches for the position that answer occupies on an ordinary UPSC paper. On this booklet that position holds a different statement altogether. On the merits the option is plainly wrong: the Reason is true. Humid heat genuinely is more uncomfortable, and the effect is measurable through the heat index and the wet-bulb temperature, which is why humidity, not temperature alone, is the standard measure of heat stress.  (c) The Assertion is not false. Evaporative cooling is the dominant route by which the human body sheds heat in hot conditions, precisely because it is the only one that does not require the surroundings to be cooler than the skin. Candidates sometimes reject the Assertion on the ground that sweating alone does not cool you — which is a real point, since sweat that drips off carries away almost nothing — but the Assertion says evaporation of sweat, not secretion of sweat, and evaporation is exactly the step that removes the heat.  (d) Both statements are true, so this option gets that far, but the explanatory arrow points the wrong way. Discomfort in humid weather does not explain why evaporating sweat cools the body; the latent heat of vaporisation explains that, and it works whatever the humidity. What high humidity does is obstruct the mechanism in the Assertion, and the resulting discomfort is the consequence. Note also that in this booklet (d) is the 'R IS the explanation' slot — the meaning that sits at (a) on a normal UPSC paper — so a candidate applying the remembered code arrives here for the wrong reason.

Concept: The human body exchanges heat with its surroundings by four routes: radiation, conduction, convection and evaporation. The first three are driven by a temperature difference and only work while the environment is cooler than the skin, which sits at roughly thirty-three to thirty-five degrees Celsius. Evaporation is different. It works on a vapour-pressure difference rather than a temperature difference, so it keeps working even when the air is hotter than the body. Sweat glands wet the skin, the water absorbs its latent heat of vaporisation from the skin and the blood beneath, and it leaves as vapour taking that heat with it. Crucially, only sweat that evaporates cools; sweat that runs off the body has done nothing. The rate of evaporation depends on how far the air is from saturation, which is why relative humidity, not temperature alone, governs how hot a hot day feels.

State PCS 2025 Q115 — Science & Technology PYQ with Answer

State PCS 2025 Q115 — Science & Technology PYQ with Answer