A gas can be compressed or expanded by pressure changes. The pressure produces a net force at right angles to the wall of the gas container (or any surface). • Students should be able to use the particle model to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure. • For a fixed mass of gas held at a constant temperature: pressure × volume = constant, p V = constant • pressure, p, in pascals, Pa • volume, V, in metres cubed, m3 • Students should be able to calculate the change in the pressure of a gas or the volume of a gas (a fixed mass held at constant temperature) when either the pressure or volume is increased or decreased.