Take a fixed mass of gas in a sealed container that cannot change size, so the volume is constant. If you raise the temperature, the pressure of the gas increases. If you lower the temperature, the pressure decreases. At GCSE you only need to describe this relationship in words.
The reason is the motion of the molecules. At a higher temperature the molecules move faster, so they hit the walls of the container more often and with greater force. The force on each square metre of wall is bigger, so the pressure is bigger. At a lower temperature the molecules move more slowly and the pressure is lower.
You can see the effect in practical demonstrations. A tightly sealed can that is heated can explode, because the pressure inside rises until the walls give way. In an Alka-Seltzer rocket the reaction makes gas in a sealed film canister and the pressure rises until the lid is forced off. An inflated balloon placed in a freezer shrinks slightly as the gas inside cools.
