Question
The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason? (2011)
Correct Option (c)
The density of water is maximum at 4°C. This unique property, known as anomalous expansion of water, is crucial. As the surface water of a lake cools, water at 4°C is denser than water at any other temperature (above 0°C). Consequently, water at 4°C sinks to the bottom of the lake. Water colder than 4°C (e.g., 3°C, 2°C, 1°C, 0°C) is less dense than water at 4°C, so it remains near the surface. When the ambient temperature drops below 0°C, this colder, less dense surface water freezes, forming a layer of ice. This ice layer then acts as an insulator, preventing further significant heat loss from the deeper water. As a result, the water at the bottom, remaining at approximately 4°C due to its higher density, stays in a liquid state, allowing aquatic life to survive.
Incorrect Options
(a) Ice is a poor conductor of heat. While this property is true and contributes to insulating the water below once a layer of ice has formed, it is not the primary reason why the water at the bottom initially remains liquid at 4°C. The anomalous density behavior of water is the fundamental cause that allows the 4°C water to settle at the bottom before freezing occurs at the surface.
(b) This statement is incorrect. In severe winter, the air temperature is significantly below 0°C, leading to continuous heat loss from the lake's surface to the colder air. If no heat were lost, the water would not freeze. The freezing process itself indicates heat loss from the water to the environment.
(d) Since statement (c) provides the correct explanation, this option is incorrect.