Gas Pressure – IGCSE Chemistry Definition

Gas pressure is the force per unit area exerted on the walls of a container by gas molecules colliding with those walls. Every gas molecule moves randomly at high speed, and each time a molecule strikes the container wall it exerts a tiny force. The combined effect of countless collisions per second on every square centimetre of the wall produces a measurable pressure.

Particle explanation

In the kinetic particle theory, gas molecules are in constant random motion, travelling in straight lines until they collide with each other or the container walls. When a molecule bounces off a wall, it transfers momentum to the wall. The greater the number of collisions per second and the harder each collision, the higher the pressure. This is why increasing the number of molecules, raising the temperature, or decreasing the volume all increase gas pressure.

Effect of temperature

When a gas is heated at constant volume, the molecules gain kinetic energy and move faster. Two things happen: the molecules hit the walls more often (more frequent collisions) and each collision transfers more momentum (harder hits). Both effects increase the pressure. This is why a sealed aerosol can may explode if heated --- the pressure inside rises as the temperature increases.

Effect of volume

If a gas is compressed into a smaller volume at constant temperature, the same number of molecules occupy less space. They hit the walls more frequently because they travel shorter distances between collisions. The increased collision rate raises the pressure.

Exam context

Paper 4 questions often present a scenario --- a sealed syringe being heated, or a tyre on a hot day --- and ask candidates to explain the pressure change using kinetic theory. Marks are awarded for identifying that molecules move faster, collide with walls more frequently and with greater force, and therefore the pressure increases. Simply saying “heat makes pressure go up” without a particle-level explanation scores zero.

Worked exam question

A sealed flask of gas is heated from 20 degC to 80 degC. The volume of the flask does not change. Explain why the pressure inside the flask increases. (3 marks)

The gas molecules gain kinetic energy and move faster [1]. They collide with the walls of the flask more frequently [1] and with greater force, so the pressure increases [1].

Common mistakes

The most frequent error is saying “the molecules expand” when heated. Molecules do not expand --- they move faster. Another mistake is forgetting to mention the walls of the container. Pressure is about collisions with the walls, not collisions between molecules. Candidates should also avoid saying “the gas expands” when the question states the volume is fixed.