How do hydrogen bonds help make ice less dense than liquid water?

Hydrogen bonds are responsible for the unique density properties of water and ice. In liquid water, molecules are associated in a flexible, polymeric structure where hydrogen bonds constantly break and reform, allowing the molecules to pack closely together.12

As water freezes, these molecules become more rigidly organized. The hydrogen bonds force the water molecules into a specific, ordered crystalline structure, forming an open, hexagonal lattice. In this arrangement, each oxygen atom is tetrahedrally coordinated with four neighboring molecules, which increases the space between them.132

This open assembly results in a lower density for ice (approximately 917 kilograms per cubic meter) compared to the more closely packed molecules in liquid water (about 1,000 kilograms per cubic meter). Because of this lower density, ice floats on the surface of water, which is essential for insulating the water beneath and protecting aquatic life during cold temperatures.12