The three states of matter are solid, liquid and gas.

• Solids have a fixed volume and usually they have a fixed shape.
• Liquids have a fixed volume but no fixed shape. They will take the shape of their container.
• Gases have no fixed volume or shape. A gas will expand to fill any container it is placed into.

The state of a pure substance at any given time will depend on the temperature.

If we take water as an example

Ice

The molecules of water are arranged close together in the ice crystal structure. Each molecule vibrates slightly but is held in place by the molecules around it.

Liquid Water

As ice receives thermal energy the molecules vibrate more. Eventually they vibrate enough to break free of the ice structure. This is melting and results in liquid water.

The water molecules are held close to each by attraction. The forces of attraction between the molecules are called "hydrogen bonds".

Water Vapour

If more thermal energy is introduced the molecules gradually gain enough energy to break away from the "hydrogen bonds" holding them together. The molecules at the surface of the water move into the air as water vapour. The molecules move freely in the air.

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