Molecular basis for the Volume Increase of Ice:
The normal pattern for most compounds is that as the temperature
of the liquid increases, the density decreases as the molecules
spread out from each other. As the temperature decreases, the
density increases as the molecules become more closely packed.
This pattern does not hold true for ice as the exact opposite
In liquid water each molecule is hydrogen bonded to approximately
3.4 other water molecules. In ice each each molecule is hydrogen
bonded to 4 other molecules.
Compare the structures of Liquid
Water and Solid Ice - Graphic Notice the empty spaces within
the ice structure, as this translates to a more open or expanded
structure. The ice structure takes up more volume than the liquid
water molecules, hence ice is less dense than liquid water.
- structures from Martin