Why water expands its volume when frozen?

Answer 1

The water molecule is polar, and if you can imagine looking at it through a super-microscope, looks like a triangle with the Oxygen at one apex and a hydrogen at the other apeces(!!). The electron-poor hydrogen part attempts to orient itself to the electron-rich oxygen part of another molecule.

In liquid water, the molecules move too fast for this orientation to take effect. .But when water freezes, the electro-attractive effect becomes important, and the molecules "line-up" into a regular crystalline structure, In ice, the distance between molecules in the structure are FURTHER apart than the mean distance between molecules in the more disordered liquid.

Answer 2

The crystal lattice of ice is a hexagonal structure in which the H2O molecules are held further apart than they are in liquid water. There are more H2O molecules per unit of volume in the liquid state than the solid state of ice. This also explains why water is denser than ice and ice floats.