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Water is a special  substance regarding the density.

There are hydrogen oxygen bonds in water.  They are special.  Water molecules come closer as the temperature reduces to 4 deg. Centigrade and then again  below 4 deg. Centigrade the water molecules separate from each other.  Thus density is highest at 4 deg. Centigrade. 

Thus water molecules in crystal water ie., ice are arranged further away from each other.  This is due to the bonds, repulsions and attractions between hydrogen and oxygen among the neighbouring molecules.  The molecules are rigidly arranged in a crystalline form.

So ice is lighter than water.

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