Water’s Density Anomaly: An Unexpected Change in Behavior

Most liquids behave in a fairly straightforward way when they are cooled: as temperature drops, their molecules move more slowly, come closer together, and the density of the liquid phase increases more or less monotonically right up to the freezing point. When they solidify, most substances form an even more compact crystal lattice, so as temperature continues to fall below the freezing point, the density of the solid usually increases a bit more.

When water is cooled down to about +4 °C, its density does increase, just like an ordinary liquid. But below +4 °C its internal structure begins to reorganize into a more “ice-like” pattern — and that’s where the strange behavior starts.

The molecules tend to arrange themselves into a tetrahedral network: the distances between neighboring molecules increase, extra “empty” spaces appear in the structure, and the liquid becomes more open and less compact. As a result, the density actually decreases, even though the water is still in the liquid state. When it finally transitions to the solid phase, the density drops even further, because the crystalline lattice of ice is more open than the liquid structure. At still lower temperatures, the density of ice, like that of most solids, begins to increase gradually again.

These changes are driven by the peculiarities of hydrogen bonding. As water cools, hydrogen bonds increasingly “lock in” fixed angles between molecules. The liquid starts to partially adopt the same tetrahedral, low-density arrangement that becomes fully developed only in the crystal structure of ice.

Schematic illustration of water’s density anomaly around 4 °C — Qualitative illustration of how water’s density reaches a maximum near +4 °C, then decreases as the structure becomes more ice-like.

Why This Matters for Life

Because of this anomaly, water in lakes and rivers behaves very differently from ordinary liquids. Ice forms on the surface, while the densest water, at about +4 °C, sinks to the bottom. The ice cover on top acts like an insulating lid: it helps trap heat, prevents the entire body of water from freezing solid, and keeps conditions in the deeper layers relatively stable.

This is why aquatic ecosystems can survive winter. Fish, plants, and other organisms remain active under the ice, even in harsh climates.

Water Is Not Completely Unique — But It’s the Champion

Water often seems like the only substance with such bizarre behavior, but it is not completely unique. Similar anomalies are known for silicon, germanium, and molten SiO₂ — network-forming materials whose chemistry can create “alien worlds” where liquid silicates play the role that water plays on Earth.

However, in terms of how strong the effect is, over what temperature range it appears, and how dramatically it influences the environment, water is in a class of its own. None of these other substances shows such a pronounced “change of behavior” right inside the liquid phase the way water does.