What causes gas pressure, and how does it change with temperature and volume?
Explaining gas pressure in terms of particle collisions, and how gas pressure changes with temperature and volume at the particle level.
A CCEA GCSE Physics answer on how gas pressure arises from particle collisions with container walls, and how the pressure changes when the temperature or volume of a fixed mass of gas is changed.
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What this dot point is asking
CCEA wants you to explain gas pressure using the particle model, and to describe how the pressure of a fixed mass of gas changes when its temperature or volume is changed. You must link the pressure to the speed and frequency of particle collisions with the walls.
The answer
What causes gas pressure
The pressure depends on how hard and how often the particles strike the walls. Anything that increases the force per collision or the number of collisions per second raises the pressure.
Temperature and pressure (constant volume)
When a fixed mass of gas is heated at constant volume:
- The particles gain kinetic energy and move faster.
- They hit the walls harder and more often per second.
- So the pressure increases.
This is why an aerosol can carries a warning not to heat it: the rising pressure could make it burst.
Volume and pressure (constant temperature)
When a fixed mass of gas is compressed (volume reduced) at constant temperature:
The particle speeds are unchanged here (temperature constant); it is the more frequent collisions that raise the pressure.
Worked example: a heated gas
Examples in context
Example 1. Car tyres on a hot day. The air inside warms up, the particles move faster, and the tyre pressure rises, which is why tyre pressures are checked when the tyres are cold.
Example 2. A bicycle pump. Pushing the plunger reduces the air volume, so the trapped particles collide more often with the walls and the pressure climbs until it is high enough to push air into the tyre.
Try this
Q1. What causes the pressure of a gas on its container walls? [2 marks]
- Cue. Particles colliding with the walls, each collision exerting a force; many collisions per second give a steady pressure.
Q2. What happens to gas pressure if a fixed mass of gas is heated at constant volume? [1 mark]
- Cue. The pressure increases.
Q3. Explain why compressing a gas at constant temperature raises its pressure. [2 marks]
- Cue. The same particles in a smaller volume collide with the walls more often per second.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksUse the particle model to explain how a gas exerts pressure on the walls of its container, and why the pressure increases when the gas is heated at constant volume.Show worked answer →
Gas particles move quickly in all directions and collide with the container walls. Each collision exerts a tiny force on the wall, and the many collisions per second produce a steady pressure.
When the gas is heated, the particles gain kinetic energy and move faster. They hit the walls harder and more often per second, so the total force on the walls, and hence the pressure, increases (the volume being fixed).
Markers reward: pressure from collisions exerting force on the walls; heating gives faster particles; harder and more frequent collisions; so greater pressure.
CCEA style3 marksA sealed syringe contains a fixed mass of gas. The plunger is pushed in to halve the volume at constant temperature. Explain, using the particle model, what happens to the gas pressure.Show worked answer →
Halving the volume packs the same number of particles into half the space.
The particles hit the walls more often per second (each unit area is struck more frequently), so the pressure increases. With the temperature constant, halving the volume roughly doubles the pressure.
Markers reward: same particles in a smaller volume; more frequent collisions per unit area; so higher pressure (roughly doubled).
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)