Water expands when it freezes due to the unique behavior of hydrogen bonding as temperature drops. Unlike most substances, which become denser when they solidify, water becomes less dense because its molecules arrange into a rigid hexagonal crystalline lattice.

In liquid water, molecules move freely and pack relatively closely together despite being held by hydrogen bonds between the positively charged hydrogen atoms of one molecule and the negatively charged oxygen atoms of another. As temperature approaches 0°C (32°F), molecular motion slows and the hydrogen bonds stabilize into a permanent geometric structure. This hexagonal lattice forces water molecules to space themselves farther apart than they were in the liquid state, increasing the overall volume by approximately 9%.

Interestingly, water reaches its maximum density at 4°C before beginning to expand slightly as it continues cooling toward freezing. The full 9% volume expansion occurs during the phase transition to ice. Because ice is less dense than liquid water, it floats—a property with profound consequences for aquatic ecosystems. The layer of ice that forms on the surface of bodies of water acts as an insulator, preventing water below from freezing solid and allowing aquatic life to survive beneath the ice during winter.

This expansion has practical implications beyond ecology. The force generated by freezing water can burst pipes and containers, and the repeated freezing and thawing of water in rock crevices causes frost weathering, gradually widening cracks and contributing to landscape erosion over time.