Water is unique among substances because it expands when it freezes. This phenomenon occurs due to the molecular structure of water and the formation of hydrogen bonds. In liquid water, molecules are closely packed and move freely. As the temperature drops, these molecules slow down and begin to form hydrogen bonds, which are attractions between the slightly positive hydrogen atoms and the slightly negative oxygen atoms.

When water reaches its freezing point, these hydrogen bonds stabilize, arranging the molecules into a crystalline structure known as ice. This structure forms a hexagonal lattice with significant empty space between the molecules. This spacious arrangement makes ice less dense than liquid water, causing it to expand by about 9%. This expansion is why ice floats on top of liquid water and why frozen water can cause pipes to burst.