Why does water expand when it freezes?

Water expands when it freezes because of the unique way its molecules arrange themselves into a solid crystalline structure.

In its liquid state, water molecules are associated in a polymeric structure, where they are linked by weak hydrogen bonds and can pack relatively closely together. However, as water reaches its freezing point, these molecules become more rigidly organized. They form an open, hexagonal lattice in which each oxygen atom is tetrahedrally coordinated with four neighboring molecules.12

This specific arrangement creates more space between the molecules than exists in the liquid phase, resulting in a less dense structure. Because of this expansion, the density of ice drops to approximately 917 kilograms per cubic metre, compared to about 1,000 kilograms per cubic metre for liquid water. This lower density is the reason why ice floats rather than sinking.12