The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason?
- Ice is a bad conductor of heat.
- Since the surface of the lake is at the same temperature as the air, no heat is lost.
- The density of water is maximum at 4°c.
- None of the statements (a), (b) and (c) given is correct.
Explanation
Option (c) is correct
- During a severe winter, the surface of a lake may freeze, but the water underneath remains in liquid form. This occurs primarily due to the anomalous behaviour of water with respect to temperature and density.
- Water reaches its maximum density at 4°C. As water cools down from higher temperatures, it becomes denser and sinks. However, when the temperature falls below 4°C, water begins to expand and becomes less dense, so it stays on the surface. This colder, lighter water eventually freezes on top, forming a layer of ice. The denser 4°C water settles at the bottom, where it stays in a liquid state. This acts as a thermal buffer, protecting aquatic life during the winter.
- Although it is true that ice is a poor conductor of heat (as in option a), this is not the main reason for the bottom water staying liquid. Option (b) is incorrect because the surface of the lake can still lose heat to the air, even if their temperatures are close.
