Water Is the Only Common Substance That Expands When It Freezes

The sight of ice cubes floating in a glass is so common we rarely question it, yet it represents a profound chemical anomaly. This behavior is rooted in the unique structure of the water molecule (H2O) and its hydrogen bonds. In liquid form, water molecules are tightly but haphazardly packed, able to slide past one another. As they freeze, these molecules arrange themselves into a highly ordered, hexagonal crystalline lattice. This rigid structure holds the molecules further apart than they were in their liquid state, creating more empty space and thus decreasing the overall density. Water is actually at its most dense at about 4° Celsius (39.2° F), just before it begins forming this expansive crystal structure.

The implications of this property are immense. The floating layer of ice on a lake or sea acts as an insulating blanket, protecting the liquid water below from frigid air and allowing aquatic ecosystems to survive through winter. If denser ice sank, bodies of water would freeze solid from the bottom up, making most aquatic life impossible in colder climates. This same expansive force is also a powerful geological agent. It is responsible for the process of frost wedging, where water seeping into rock crevices freezes, expands, and widens the cracks, eventually breaking massive boulders apart over time and shaping landscapes.