As volume increases, pressure?
Easy points

•Each time they collide with the walls they exert a force on them. More collisions mean more force, so the pressure will increase. When the volume decreases, the pressure increases. This shows that the pressure of a gas is inversely proportional to its volume.

Boyle’s law

Decreasing the volume of a gas increases the pressure of the gas. An example of this is when a gas is trapped in a cylinder by a piston. If the piston is pushed in, the gas particles will have less room to move as the volume the gas occupies has been decreased.

A piston closes the end of a cylinder with some gas molecules inside. The pressure applied to the piston is doubled and the volume inside the cylinder halfs.

As the pressure applied to a piston is doubled, the volume inside a cylinder is halved

Because the volume has decreased, the particles will collide more frequently with the walls of the container. Each time they collide with the walls they exert a force on them. More collisions mean more force, so the pressure will increase.

When the volume decreases, the pressure increases. This shows that the pressure of a gas is inversely proportional to its volume.

This is shown by the following equation - which is often called Boyle’s law. It is named after 17th century scientist Robert Boyle.

P1V1 = P2V2

where:

P1 is the initial pressure

V1 is the initial volume

P2 is the final pressure

V2 is the final volume

It can also be written as:

pressure1 × volume1 = pressure2 × volume2

Note that volume is measured in metres cubed (m3) and temperature in kelvin (K).

It means that for a gas at a constant temperature, pressure × volume is also constant. So increasing pressure from pressure1 to pressure2 means that volume1 will change to volume2, providing the temperature remains constant.