The phenomena of anomalous expansion of water help marine animals to live inside a lake even when the surface of the lake freezes. Similar to all other liquids, volume of a given amount of water decreases with temperature until its temperature reaches 4 °C. Due to the anomalous behavior, the volume increases when the water is cooled below 4 °C and therefore the density decreases. When the temperature of surface water of lake reaches at 4 °C it sinks to the bottom as it becomes denser and water from bottom moves upward to the surface. When the temperature of bottom of lake reaches 4°C the circulation of water stops and at 0°C the surface water freezes. As the ice is less dense than water it floats on the surface of lake. This frozen water acts as an insulator of heat and check the further heat to escape. The water at the bottom of lake remains at 4 °C (constant temperature) as there is no circulation of water. Hence, marine animals can easily survive deep inside the lake even when the surface of the lake freezes.
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The density of waHC Verma - Concepts of Physics Part 2