Why is ice less dense than liquid water?

Ice is less dense than liquid water because of the specific way its molecules arrange themselves upon freezing. In the liquid state, water molecules are associated in a polymeric structure where they are more closely packed together.12

When water freezes, the molecules form a highly ordered, rigid crystalline structure. In this solid state, each oxygen atom is tetrahedrally coordinated with four neighboring molecules, creating an open, hexagonal lattice. This configuration increases the space between the molecules compared to the liquid phase.32

Because of this open assembly, the density of ice is approximately 917 kilograms per cubic meter, while liquid water is about 1,000 kilograms per cubic meter. This lower density is why ice floats, allowing it to form an insulating layer on the surface of bodies of water that protects aquatic life during cold weather.13