Physics – 2.2.1 Thermal expansion of solids, liquids and gases | e-Consult

The volume of water increases when heated because the water molecules gain kinetic energy and move further apart. Water molecules are held together by hydrogen bonds, which are relatively strong intermolecular forces. At lower temperatures, these hydrogen bonds restrict the movement of the molecules, resulting in a smaller volume.

As the temperature increases, the water molecules gain kinetic energy and vibrate more vigorously. This increased vibration overcomes some of the hydrogen bonds, allowing the molecules to move further apart. The increased separation of molecules leads to an expansion of the water, resulting in an increase in volume. The hydrogen bonds are still present, but their influence is reduced by the increased thermal energy. The expansion of water is somewhat unusual compared to other liquids, as it expands upon heating up to 4°C, and then contracts above that temperature.

The strength of the hydrogen bonds is crucial. While strong, they are not strong enough to prevent significant expansion when the temperature rises sufficiently. The increased kinetic energy overcomes the attractive forces, leading to a net increase in volume.