This step looks at the behaviour of a fixed quantity of gas under varying pressure, volume and temperature.

The kinetic theory model can be used to explain the properties of solids, liquids and gases by considering the molecules they are made from and their motion.

For gases, the kinetic theory model explains that gas pressure is caused by the collisions between the particles and their container. This is called the outward pressure and is usually greater than normal atmospheric pressure outside the container.

Examples of this can be found in balloons and car tyres.