We all know that water has maximum density at 4 degree C.

 

What molecular property of water can explain this ?

 

 

The melting point of ice is 0 °C (32 °F, 273.15 K) at standard pressure, however, pure liquid water can be supercooled well below that temperature without freezing if the liquid is not mechanically disturbed. It can remain in a fluid state down to its homogeneous nucleation point of approximately 231 K (−42 °C).

 

http://en.wikipedia.org/wiki/Properties_of_water

We all know that water has maximum density at 4 degree C.

 

What molecular property of water can explain this ?

 

 

Without knowing specifics, it seems that water molecules, when forming a crystal lattice, are farther apart than when free floating.