UPSC Prelims 2011 · GS Paper I

Question 50: Science and Technology

The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason ?

aIce is a bad conductor of heat

bSince the surface of the lake is at the same temperature as the air, no heat is lost

cThe density of water is maximum at 4°C

dNone of the statements (a), (b) and (c) given above is correct

AnswerThe answer is (c).

Water is densest at 4°C. As a lake cools in winter, the surface water cools, becomes denser and sinks, until the whole lake is at 4°C. Cooling it further makes the surface water lighter, so it stays on top and freezes, while the water at 4°C remains at the bottom and stays liquid. Ice is also a poor conductor of heat, which slows the loss of heat from below, but the density maximum at 4°C is the main reason.

Why each option is right or wrong

(a) A real effect that slows further freezing, but it is not the main reason the bottom stays liquid.
(b) Heat is lost from the surface; it is not true that none is lost.
(c) Right: density is greatest at 4°C, so the 4°C water settles at the bottom.
(d) Option (c) is correct, so this fails.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2011, General Studies Paper I, Series A. The paper on upsc.gov.in.