Question

why density of water is maximum at 4C

Answer 1

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The maximum density of water occurs at 4 °C because, at this temperature two opposing effects are in balance.The thermal energy (Kinetic Energy) of water is high enough that Hydrogen bonds cannot hold the molecules for long. They get knocked around by other molecules and this energy imparted is enough to break the hydrogen bonds between Oxygen atom of one molecule and Hydrogen atom of another molecule. 

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Answer 2

water performsanomalousbehaviour.According to this , unlike other substances, water expands on decreasing the temperature . As a result, the volume is maximum at 0 degree celsius.
On incresing the temperature from 0 to 4 degree density increases.
Therefore, the density of water is maximum at 4 degree.