# The behaviour of gases

## Charles’ Law

Charles’ Law states that if a gas is heated up and the pressure does not change, the volume will. So, for a fixed mass of gas at a constant pressure, volume ÷ temperature will remain the same.

## Boyle’s Law

Boyle’s Law states that for a gas at a constant temperature, pressure × volume is also constant. So, increasing pressure means that volume will decrease providing that the temperature remains constant.

### What happens if we change the temperature of a fixed volume of gas?

• When the temperature is increased, the gas particles move faster and the collisions become harder and more frequent. This means that the pressure also increases.
• When the temperature is decreased, the gas particles move more slowly and the collisions are less hard and less frequent. This means that the pressure also decreases.

Eventually at absolute zero, which is -273°C, the molecules stop moving and the gas does not exert any pressure at all. This temperature is known as absolute zero.

The average kinetic energy of the particles in a gas is directly proportional to the temperature of the gas.