Expansion and Contraction

Expansion and Particles

Solids, liquids and gases do not always change state if they gain or lose heat energy. Whether or not a change of state occurs depends on how much heat is gained or lost. Even if no change of state occurs, a material is still affected - it will expand or contract (get smaller).

The particle model can explain expansion and contraction. When a solid or liquid is heated, its particles vibrate more rapidly and push each other further apart, so the substance takes up more space, or has expanded. In a gas, particles move with higher speed and push harder against anything they come in contact with. If the gas were in a balloon, the balloon would expand.

When a solid or liquid is cooled, its particles vibrate less rapidly and can move closer together and occupy less space.

Expansion of solids

Different solids expand at different rates. Building materials like concrete and steel can expand and so consideration must be given when designing buildings and other structures. Expansion gaps must be left in bridges and railway tracks, otherwise they may buckle as they expand in warm weather. You may also have noticed that power lines sag more on hot days due to expansion.

Expansion of liquids

In general, liquids expand much more than solids when heated. Care must be taken to leave space for liquids to expand into when filling containers such as petrol and liquid petroleum gas (LPG) tanks. A car radiator contains water that is used to cool the engine. This water expands when it absorbs engine heat, and may overflow into an expansion chamber.

Thermometers are used to measure temperature. They contain a liquid - mercury or coloured alcohol - in a bulb connected to a narrow tube that makes the liquid rise noticeably when heated.

Expansion of gases

Gases react to changes in temperature more than solids and liquids do. A gas will try to expand if heated, but may be prevented from doing so by its container. Since the particles of a gas will diffuse to completely fill a container, it’s not always quite correct to say that gases contract to take up less space. Instead we say that a gas exerts less pressure when it is cooled.