Advanced #23

Thermal expansion of water decreases increasingly at low temperatures

Upon cooling below 4 degrees Celsius, thermal expansion becomes negative -- water expands upon cooling.

Scientific Explanation

For most liquids, the thermal expansion coefficient is positive and changes only modestly with temperature. Water behaves fundamentally differently: its expansion coefficient decreases steadily with falling temperature, passes through zero at 3.984 degrees Celsius, and becomes negative below that point. This means water below 4 degrees Celsius expands as it cools rather than contracting.

This negative expansion coefficient is a direct consequence of increasing ice-like structural ordering at low temperatures. The colder the water gets, the more molecules arrange themselves in tetrahedral configurations resembling the crystal structure of ice. Because these structures occupy more space than the denser, disordered arrangement found at higher temperatures, the volume increases even as temperature drops.

In the supercooled regime (below 0 degrees Celsius), this effect becomes dramatic. The expansion coefficient grows ever more negative, reflecting the rapid approach toward ice-like structure.

Thermal Expansion Coefficient vs Temperature Line chart showing the thermal expansion coefficient of water versus temperature. The coefficient is negative below 4 degrees C, passes through zero at 4 degrees C, and becomes increasingly positive at higher temperatures. A horizontal dashed line marks the zero crossing. α (10⁻⁴ K⁻¹) Temperature (°C) α = 0 +6 0 -6 -10 0 10 20 30 40 Negative α ~4 °C Thermal Expansion Coefficient of Water
The thermal expansion coefficient of water passes through zero at about 4 degrees Celsius and becomes negative at lower temperatures.

Everyday Relevance

The negative thermal expansion at low temperatures explains why water in pipes and containers builds up significant pressure as it approaches freezing. Water expands before becoming ice — the last few degrees above the freezing point already involve a volume increase. This is one reason why water pipes can burst in winter: not just the ice itself, but the expanding cold water generates pressure.

This behavior is also crucial for lake mixing. In autumn, surface water cools and sinks until it reaches 4 degrees. Water that cools further remains at the surface because it expands and becomes lighter.