UPSC Prelims 2011 Q50: Science & Technology – Physics
The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid.
What is the reason ?
- (a) Ice is a bad conductor of heat
- (b) Since the surface of the lake is at the same temperature as the air, no heat is lost
- (c) The density of water is maximum at 4°C
- (d) None of the statements (a), (b) and (c) given above is correct
Answer: (c) The density of water is maximum at 4°C
Explanation
Why a lake's bottom water stays liquid under frozen surface ice
This is a classic property of water's density anomaly.
| Option | Verdict | Reason |
|---|---|---|
| (a) Ice is a bad conductor of heat | Incorrect | True in general, but it is not the reason the bottom stays liquid; it's about density, not insulation alone |
| (b) Surface at same temperature as air, no heat lost | Incorrect | This is factually wrong; the surface IS colder (at freezing point) while deeper water is warmer |
| (c) Density of water is maximum at 4°C | Correct | Water is densest at 4°C, so this layer sinks to the bottom; colder water (closer to 0°C, less dense) stays near the surface and freezes, insulating the warmer, denser water below |
| (d) None of the above is correct | Incorrect | Option (c) is correct, so this is wrong |
Water's density anomaly:
| Point | Detail |
|---|---|
| Max density | At 4°C |
| Effect | Densest water sinks, keeping the bottom liquid and relatively warm |
| Result | Ice forms only at the surface, insulating the water below |
Answer: (c) The density of water is maximum at 4°C.
Why each option is right or wrong
- (a) Incorrect. Ice's poor conductivity helps insulate, but the root cause of the bottom staying liquid is water's density behavior at 4°C.
- (b) Incorrect. This statement is factually false — the surface is colder than the air-exposed ice layer, not at the same temperature as the air with 'no heat lost'.
- (c) CORRECT. Water's maximum density at 4°C makes that layer sink, keeping the bottom liquid while ice forms only on top.
- (d) Incorrect. Since (c) is a valid, correct explanation, 'none of the above' cannot be right.
Key point
Water is densest at 4°C, so that layer sinks to the bottom of a lake, keeping it liquid even when the colder, less-dense surface water freezes into ice.
Remember
Lake stays liquid at the bottom because water's density is maximum at 4°C, so that layer sinks beneath the colder, less dense, freezing surface water.
Common mistakes
- Attributing the phenomenon only to ice's insulating property, missing the density anomaly.
- Not knowing water's density peaks at 4°C, unlike most liquids that are densest when coldest.
- Assuming the lake surface and air are at the same temperature with no heat exchange.
Exam tip
Water is unusual: it's densest at 4°C, not at 0°C. This is why lakes freeze top-down, keeping life possible underneath.
Sources
- NCERT Class XI Physics — Thermal Properties of Matter (anomalous expansion of water)
- H. C. Verma — Concepts of Physics, Volume 2 (Heat and Temperature)