A fixed volume of gas

1. A gas has a constant volume when it is sealed inside a rigid container whose walls cannot move, such as a steel gas cylinder, a sealed metal can or a stoppered glass flask.
2. Because the container is sealed, the number of molecules inside stays the same as well.
3. With the volume and the number of molecules both fixed, the only way the pressure can change is through a change in the speed of the molecules.
4. The speed of the molecules is set by the temperature, so at constant volume the pressure of a gas depends on its temperature.

Raising the temperature

1. Heating the container transfers energy to the gas and increases the kinetic energy of its molecules.
2. The molecules move faster on average, which is what a rise in temperature means.
3. Faster molecules cross the container in less time, so each one hits the walls more often.
4. Faster molecules also hit the walls harder, so each collision exerts a greater force.
5. More frequent collisions and a greater force per collision together increase the total force on each square metre of wall.
6. The pressure of the gas therefore increases, even though its volume and the number of molecules have not changed.
7. You only need to explain this relationship in words, so no calculation is needed for a change of temperature at constant volume.

Lowering the temperature

1. Cooling the container transfers energy away from the gas, so its molecules move more slowly on average.
2. Slower molecules hit the walls less often and less hard.
3. The total force on each square metre of wall falls, so the pressure decreases.
4. Warming the gas back to its starting temperature returns the pressure to its starting value, because nothing has left the sealed container.
5. The pattern holds in both directions: at constant volume, a higher temperature gives a higher pressure and a lower temperature gives a lower pressure.

Constant volume in everyday life

1. A car tyre holds a fixed amount of air in a casing that barely stretches, so its pressure rises during a long motorway drive as the tyre warms up.
2. This is why drivers are told to check tyre pressures when the tyres are cold, because a reading taken straight after a journey is higher than normal.
3. Aerosol cans warn you not to leave them in temperatures above $50{}^{\circ }C$, because heating the sealed can raises the pressure inside and could make it burst.
4. A gas cylinder for a barbecue is stored in the shade for the same reason.
5. A plastic bottle sealed on a warm day and then put in a fridge is squashed slightly, because the pressure of the cooled air inside falls below the pressure of the air outside.