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 occurs.

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 - Graphi 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.