Q. The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason? (UPSC Prelims 2011)
Answer:
The density of water is maximum at 4°C
Notes: The correct answer is
The density of water is maximum at 4°C. This phenomenon is known as the
anomalous expansion of water, which is crucial for the survival of aquatic life in cold climates.
- Maximum Density at 4°C (Correct): Unlike most liquids that contract and become denser as they cool, water reaches its maximum density at 4°C. As the surface water cools toward 4°C, it becomes heavier and sinks to the bottom. Once the entire body of water reaches 4°C, further cooling of the surface water (below 4°C) makes it less dense.
- Layering Effect: Because water colder than 4°C is lighter, it stays at the surface until it freezes into ice at 0°C. The denser, warmer water (4°C) remains trapped at the bottom, protected by the layers above.
- Insulation: While option [A] is a true scientific fact (ice is a poor conductor), it is a secondary reason that helps maintain the state. The primary physical reason why the bottom remains liquid instead of freezing solid from the bottom up is the density anomaly.