What is the reason for anomalous expansion of water?
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Water has a very special property of anamalous expansion, unlike any other substances or liquids. All substances expand when they are heated or when their temperature increase and the density decreases. Water has also this property. But when it looses heat and reduces the temperature and reaches 4 degree celsius its density becomes highest. Further decrease in temperature results in the expansion of its volume. Thus between 0 and 4 degree temperature water expands with decrease in temperature. Between 0 and 4 degree Celsius, its density decreases with decrease in temperature . This special naturral property of water gives a good protection cover for the acquatic lives in the polar region where temperaturre falls below freezing temperature. The outer layer of water becomes ice and floats over water as a special cover of safety. Ice is a bad conductor of heat.The heavy water at highest density at 4 degree sinks down and keeps its acquatic beings in safest position. Moreover water is also a bad conductor of heat. Thus it is the special property of water that keeps the acquatic life in safe from below feezing temperature.
Neela answers the "what" of the expansion very well, but I have a feeling you are also looking for the "why". When most substances cool from a liquid to a solid state, the volume decreases because the molecules are slowing down. They compress, and take up less space. Some, water notably, instead form into a crystalline structure when they freeze. Ice has a crystalline structure, which takes up MORE space than the water it comes from. It is also therefore less dense (same weight of water in a bigger volume), and floats on the surface of a lake, pond, river, etc. If this fact were not true, the earth could very well be a block of ice, as the frozen lakes and such would not melt easily--they would freeze from the bottom up, instead of the top down, and it would very difficult for them to melt.
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