Expansion and contraction

Substances expand (increase in size) when they get warmer, and they contract (decrease in size) when they get cooler. This property can be useful. For example:

  • Thermometers work because the liquid inside them expands and rises up the tube when it gets hotter.
  • Metal parts can be fitted together without welding, using shrink fitting. The slideshow shows how this works:
Image 1 of 3: The rod is too big to fit through the hole.

The rod is too large to fit through the hole

Bridge expansion joint.
Expansion joints like this allow bridges to expand and contract without damage

The expansion and contraction of materials can also cause problems. For example, bridges expand in the summer heat and need special joints to stop them bending out of shape.

What do the particles do?

When substances expand or contract, their particles stay the same size. It is the space between the particles that changes:

  • the particles in a solid vibrate more when it is heated, and take up more room
  • the particles in a liquid move around each other more when it is heated, and take up more room
  • the particles in a gas move more quickly in all directions when it is heated, and take up more room
Compares particles in a solid when cold and when hot. When the particles are cold, they are closer together and vibrate less. When they are hot, they vibrate more and are further apartThe space between particles changes when a substance expands (or contracts)

Remember, the particles themselves never expand.