Why has the water maximum density at 273.15 K n not at sum other point?

Dear Student,

Water has maximum density at 277 K i.e. 4º C and not 273.15 K. Maximum density is obtained only at this temperature and not other temperatures because if the temperature is lowered, the water will solidify and form ice. Ice has greater hydrogen bonds as hydrogen bonds become stronger with decrease in temperature. Now, these hydrogen bonds forms a open cage like structure which has empty spaces between it. This increases the volume of ice and thus, decreases its density. If temperature is increased, then the hydrogen bonds starts to break which decreases the volume and thus increases the density. This maximum density temperature is 277 K. After this temperature, intermolecular repulsion increases which further increases the volume and thus decreases the density. 

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273.15 K = 0C
Hence, the water will be converted into ice at the given temperature and its density is maximum at that temperature.
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 The answer that you're looking for is water has very strong hydrogen bonding that creates structure in the molecules. Liquids that do not possess hydrogen bonding generally exhibit increasing density as temperature decreases as molecules pull tighter together. At temps below 1C, water begans to exhibit orientation due to hydrogen bonding. This network expands the volume of the liquid resulting in lower density. 
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