Q.3 (viii) (a) What is meant by anomalous behaviour of water ?
(b) Explain the role of anomalous behaviour of water in preserving aquatic life in cold climate.
Best answer
Answer: (a) Between 0 ºC and 4 ºC water contracts on heating and expands on cooling, so its density is maximum at 4 ºC; (b) this keeps the water below the ice layer at about 4 ºC, so aquatic animals survive.
(a) Normally a substance contracts on cooling. Water, however, contracts when heated from 0 ºC to 4 ºC and expands when cooled from 4 ºC to 0 ºC. Its volume is least and density greatest at 4 ºC. This unusual behaviour is called the anomalous behaviour of water.
(b) In very cold weather the surface water cools first. When it reaches 4 ºC it becomes densest and sinks, so the warmer, lighter water below comes up; this continues till all the water is at 4 ºC. Further cooling of the surface water makes it lighter, so it stays on top and freezes, forming a layer of ice. Ice is a poor conductor of heat, so it insulates the water below and the water under the ice stays at about 4 ºC. Fish and other aquatic animals can therefore live through the winter.
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