Particle motion in gases — AQA GCSE Physics
Test yourself on Particle motion in gases with AQA GCSE practice questions.
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Particle motion in gases explained
Seal a gas in a rigid container so the volume cannot change, then warm it.
Read the full explanation
The mean kinetic energy of the molecules rises, so they move faster. Two things follow. Each molecule hits a wall harder, exerting a greater force during each collision, and each molecule reaches the walls more often, so collisions with the walls become more frequent. Pressure is the total force on the walls divided by their area, and both contributions have gone up, so the pressure rises. Cooling reverses both steps and the pressure falls. This is why an aerosol can carries a warning against heating: the volume is fixed, so the pressure inside climbs until the can may burst.
Your focus
- Explain, in terms of molecular collisions, why the pressure of a sealed gas rises when it is heated.
- Predict the effect on pressure of cooling a gas held in a rigid container, and justify the prediction.
- Explain why aerosol cans carry a warning against storing them near a heat source.
Particle motion in gases exam tips
Marking Points
- one mark for stating that the mean kinetic energy, or mean speed, of the molecules increases
- one mark for stating that the molecules exert a greater force on the walls during each collision
- one mark for stating that the frequency of collisions with the walls increases
- one mark for the conclusion that the pressure of the gas increases
Examiner Tips
- 💡Four marks usually means four steps: kinetic energy, force per collision, frequency of collisions, pressure. Write them in that order.
- 💡Say collisions with the walls of the container, not just collisions, so the examiner can award the mark.
- 💡State clearly that the volume and the number of molecules are unchanged, so the examiner knows you have not confused this with a volume change.
Common Mistakes
- saying the molecules themselves expand when heated
- claiming the pressure rises because there are more molecules, when the container is sealed
- writing that the gas pushes harder because it is hotter, with no mention of collisions with the walls
- describing collisions between molecules only, and never collisions with the container walls