Question

In very cold places, when the temperature becomes very low, the surface of lakes freezes while
the water inside does not. As a result, fish and other animals can still live in the lake. Can you tell
why the water inside the lake does not freeze? (Hint: Is ice a good conductor or insulator?)
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Answer 1

Answer:

Ice is a crystal, which means it has a regular pattern with spaces in between molecules.

The spaces in the crystal are larger than the spaces between molecules in the liquid.

More space = less dense, so ice is less dense than water.

So although the air is freezing cold, the water isn't freezing yet.

Answer 2

Answer:

This process is known as Anomalous expansion of water. In cold weather, when the atmospheric temperature starts falling below 0°C, water at the surface of a pond initially at temperature above 4°C, begins to radiate heat to the atmosphere, so the temperature of the water near the surface starts falling. When the temperature of the water falls below 4°C, water contracts and density increases and therefore it sinks to the bottom. This continues till temperature of the entire water reaches to 4°C result in the expansion of water. Now, further cooling of top layer below 4°C results in expansion of water and so it's density decreases. As, a result water doesn't stink further, but it remains on the surface of the water. When the temperature of atmosphere falls below 0°C, water on the surface loses further heat to the atmosphere and gradually freezes into ice, but water below the ice in contact with it will be at 0°C. Since, ice is a poor conductor of heat, so ice now prevent the flow of heat from water of pond to the atmosphere. Thus, temperature of water in contact with ice is at 0°C, while the temperature of water layers below the ice gradually increase to 4°C, as a result fish and other aquatic creatures remain alive in water of the pond though the surface of the pond has been freeze.

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Extra Info

Anomalous expansion of water is the unusual expansion of water when it is cooled from 4°C to 0°C. Normally all substances contracts on cooling but, water expands thus it is called unusual.