A fixed quantity of gas is trapped in a container at constant temperature. Explain, using kinetic theory, why the pressure of a gas increases when its volume is decreased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a gas is compressed into a smaller volume at constant temperature, the pressure of the gas increases. This is known as the pressure-volume relationship.
Model answer (4 marks)
Gas particles move in all directions and collide with the walls of the container, producing pressure.
When the volume is decreased, the same number of particles are confined to a smaller space, so they are closer together.
This reduces the distance each particle travels before hitting a wall, increasing the frequency of collisions.
More frequent collisions give a greater force per unit area on the walls, so the pressure rises.
When the volume is decreased, the same number of particles are confined to a smaller space, so they are closer together.
This reduces the distance each particle travels before hitting a wall, increasing the frequency of collisions.
More frequent collisions give a greater force per unit area on the walls, so the pressure rises.
Examiner tips
- Use the phrase "collide with the walls" to show kinetic theory knowledge.
- Explain the chain: smaller volume → particles closer → more collisions → higher pressure.
- Show the link between collision frequency and force per unit area.
- Keep the answer concise – 4 points, one sentence each.
Common mistakes
- Failing to mention collisions with the walls as the source of pressure.
- Confusing temperature change with volume change – the temperature is constant.
- Using vague terms like "more particles" instead of "more frequent collisions”.
Mark scheme (4 marks)
- Gas particles collide with the walls of the container, producing pressure
- When volume decreases, the particles are closer together / in a smaller space
- The frequency of collisions with the walls increases
- More frequent collisions mean a greater force per unit area, so pressure increases
Key terms in this question
pressure · volume · kinetic theory
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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