Advanced #24

Thermal expansion of water increases with rising pressure

The thermal expansion coefficient of water increases unusually strongly with pressure.

Scientific Explanation

For most liquids, the thermal expansion coefficient decreases under pressure: compression packs the molecules more tightly, leaving less room for expansion when heated. Water shows the opposite behavior — its expansion coefficient increases with pressure.

This anomalous behavior is linked to the hydrogen bond network. Under normal conditions, the open, tetrahedral structure causes water to become denser when heated (negative thermal expansion below 3.98 degrees Celsius). Under pressure, the tetrahedral ordering is disrupted and partially broken. The resulting less structured liquid behaves more “normally” in the sense that it exhibits a larger positive expansion coefficient.

Put simply: pressure eliminates the anomalous compaction that hydrogen bonds cause at low temperatures. As a result, normal thermal expansion dominates more strongly, and the overall expansion coefficient rises.

Thermal Expansion Coefficient vs Pressure for Water Line chart showing how the thermal expansion coefficient of water increases with pressure, an anomalous behavior since most liquids show decreasing thermal expansion under pressure. Expansion coeff. (10⁻⁴ K⁻¹) 8 5 2 0 100 200 300 Pressure (MPa) Water (25 °C) Typical liquid Thermal Expansion vs Pressure
Thermal expansion coefficient of water and a typical liquid under pressure.

Everyday Relevance

For oceanography, this anomaly is significant: in the depths of the oceans, where high pressures prevail, water behaves differently in terms of thermal expansion than it does at the surface. This influences the thermohaline circulation — the great global current patterns that transport heat and nutrients around the planet. Precise ocean models must account for this pressure dependence of the expansion coefficient to correctly represent heat transport in the deep ocean.