Anomalous Expansion of water

 

Why does water expand when it freezes which we call Anomalous expansion of water.

 Generally liquids expand on heating and contracts on cooling. Water however shows an exceptional behaviour below 4^oC .If we cool water at room temperature we find that it goes on contracting but as the temperature falls below 4^oC it begins to expand instead of contracting, conversely if water is heated from 0^oC to 4^oC instead of expanding it contracts.

 

  Water has maximum density at 4^oC this is because of hydrogen bonding , In water, each hydrogen nucleus is covalently bound to the central oxygen atom by a pair of electrons that are shared between them and thus act as a electric dipole and  the negatively charged Oxygen of one molecule gets attracted to positively charged hydrogen atom of another water molecule to form a hydrogen bond.

 

  These hydrogen bonds are compact at 4^oC and hence maximum density ,but as we freeze further to 0^oC water expands and at 0^oC Ice forms crystals having a hexagonal lattice structure with long bonds and space which are less compact. Therefore water expands as it cools towards 0^oC