Water Is the Only Common Substance That Expands When It Freezes

Unlike almost every other substance, which contracts and becomes denser as it solidifies, water plays by its own rules. The secret lies in the V-shape of the water molecule (H₂O) and the powerful hydrogen bonds that form between them. As water cools towards its freezing point, the molecules slow down, allowing these bonds to lock them into a highly ordered, hexagonal crystal lattice. This rigid structure holds the molecules farther apart than when they were jumbling past one another in their liquid state, causing a decrease in density of about 9 percent.

This density anomaly is the reason ice cubes float in a glass and massive icebergs drift in the ocean. This property is not just a curiosity; it is essential for life as we know it. If ice were denser than water, it would sink. In winter, lakes, rivers, and even oceans would freeze solid from the bottom up, creating a barren, lifeless environment. Instead, the floating ice forms a protective, insulating layer on the surface, allowing the liquid water below to remain habitable for aquatic organisms through the coldest months. This same expansive force is also powerful enough to crack boulders and burst pipes.