| MadSci Network: Physics |
Water expands when it freezes because the water molecules crystallize in such a way that the distance between molecules in the crystal is greater than the average distance between water molecules in their liquid state.
It may surprise you to know that ice does contract as it gets colder, just as any other solid does. As the temperature drops, and the bonding energy decreases, the molecules move closer to one another. So extremely cold ice takes up less space than ice at the freezing point.