Water expands when it freezes because its molecules arrange themselves into a structured hexagonal lattice that occupies more space than when in the liquid state. This crystalline arrangement, held together by hydrogen bonds, creates open spaces between molecules, making ice about 9% less dense and thus more voluminous than liquid water. As a result, ice expands as it forms, which is unusual compared to most substances that contract upon freezing.

Water molecules have a V-shape with positive and negative sides, causing them to form hydrogen bonds that create this open lattice structure during freezing. This structure forces the molecules apart, increasing the volume of ice compared to liquid water. This expansion explains why ice floats on water and causes phenomena such as pipe bursting when water freezes in confined spaces.