Heating Solids, Liquids and Gases
When you add thermal energy to a solid, liquid or a gas, then you are making the particles in that matter move faster as each particle receives more kinetic energy.
When this happens, solid particles have larger vibrations about a fixed point, liquid particles move around each other faster, and gas particles move faster in straight lines until they collide with themselves or the walls of a container.
As the particles are moving faster, they will naturally take up more space, and so the volume of the solid, liquid or gas will increase. We say that the material has 'expanded' due to being heated.
Note that the actual particles themselves do not expand! They simply take up a larger volume.
GCSE Physics Keywords: Solid, Liquid, Gas, Pressure, Volume