Advanced #22

Water has a low expansion coefficient

The thermal expansion coefficient of water is unusually low.

Scientific Explanation

The thermal expansion coefficient measures how much a substance’s volume increases when heated. For water at 25 degrees Celsius, this value is only about 2.57 times 10 to the power of minus 4 per Kelvin — roughly four to five times smaller than for typical organic liquids such as ethanol or acetone.

The explanation lies once again in hydrogen bonds. In most liquids, heating simply causes molecules to vibrate more strongly and occupy more space. In water, there is a counteracting effect: warming disrupts ice-like open structures and enables a more compact molecular arrangement. This densification partially compensates for the normal thermal expansion, resulting in a much smaller net expansion.

Notably, the expansion coefficient of water is strongly temperature-dependent. At 4 degrees Celsius it is exactly zero (where the density maximum lies), and below this temperature it becomes negative — water actually expands upon cooling.

Thermal Expansion Coefficient Comparison Bar chart comparing the thermal expansion coefficient of water at 25 degrees C with ethanol, acetone, and mercury. Water has a much lower value than organic liquids. α (10⁻⁴ K⁻¹) 0 5 10 14 2.57 Water 11.2 Ethanol 14.3 Acetone 1.82 Mercury Thermal Expansion Coefficient at 25 °C
Thermal expansion coefficients compared. Water expands far less than organic solvents upon heating.

Everyday Relevance

The low expansion coefficient has practical importance. In heating systems and cooling circuits that use water, volume changes during temperature fluctuations are relatively small, keeping system pressure more stable. In biological systems, this property is advantageous as well: body fluids change volume very little with temperature, which helps maintain the stability of cells and organs.

Interactive Simulation

0.01.63.14.76.37.9Expansion (%)Water0.414%Ethanol2.240%Mercury0.362%Acetone2.860%
20 Δ°C