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
In a gas the molecules move continuously in every direction, with no preferred direction and a wide spread of speeds, changing direction whenever they collide with one another or with the walls.
Read the full explanation
This is what random motion means. Temperature is a measure of the average kinetic energy of those molecules, not of any one molecule and not of the total. Raise the temperature and the mean kinetic energy rises, which means the mean speed rises too, since the molecules do not change mass. Cool the gas and the mean kinetic energy falls, heading towards zero at about -273 °C. Nothing about the molecules themselves changes size when a gas is heated; only how fast they move changes.
Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas.
Seal a gas in a rigid container so the volume cannot change, then warm it. 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.
Students should be able to: explain how the motion of the molecules in a gas is related to both its temperature and its pressure explain qualitatively the relation between the temperature of a gas and its pressure at constant volume.
Pressure comes from collisions: molecules strike the walls of the container and rebound, and the total force from all those impacts, divided by the wall area, is the pressure. Two factors set it, how hard each impact is and how many impacts happen each second. Temperature controls both, because temperature is a measure of the mean kinetic energy of the molecules. Hold the volume constant and raise the temperature: the molecules move faster, so they hit harder and hit more often, and the pressure rises.
Your focus
- Describe the motion of gas molecules in terms of direction, speed and collisions.
- Explain what happens to the mean kinetic energy and mean speed of gas molecules when the gas is cooled.
- Compare the mean kinetic energy of the molecules in two samples of the same gas held at different temperatures.
Show all 9 objectives
- 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.
- Explain how gas pressure arises from collisions between molecules and the container walls.
- Describe the chain of reasoning from a temperature rise to a pressure rise at constant volume, in four linked steps.
- Predict what happens to the pressure of a fixed volume of gas when it is cooled, and justify the answer in terms of collisions with the walls.
Particle motion in gases exam tips
Marking Points
- one mark for describing the motion as random, in all directions and at a range of speeds
- one mark for stating that temperature is a measure of the average kinetic energy of the molecules
- one mark for stating that increasing the temperature increases the mean kinetic energy, or the mean speed, of the molecules
- one mark for stating that the average, rather than the total, kinetic energy is what temperature measures
- 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
- one mark for stating that pressure is caused by molecules colliding with the walls of the container
- one mark for linking a rise in temperature to a rise in the mean kinetic energy of the molecules
- one mark for stating both that each collision exerts a greater force and that collisions become more frequent
- one mark for concluding that the pressure increases when the volume is held constant
Examiner Tips
- 💡Use the word mean or average every time you mention kinetic energy in a gas answer; examiners look for it.
- 💡Mean speed is accepted as an alternative to mean kinetic energy, so use whichever you can explain clearly.
- 💡Describe random motion in two parts, all directions and a range of speeds, to secure the mark.
- 💡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.
- 💡Start every gas explanation with the sentence that pressure is caused by collisions with the container walls; everything else hangs off it.
- 💡Check which quantity the question holds constant and say so explicitly in your answer.
- 💡Qualitative means no calculation is needed, but the chain of reasoning still has to be complete to score.
Common Mistakes
- saying all the molecules in a gas travel at the same speed
- claiming the molecules themselves expand or grow when the gas is heated
- using the word heat as a synonym for temperature in the explanation
- saying the molecules move faster because they contain more heat, rather than because their kinetic energy has increased
- 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
- saying the molecules gain more energy without ever mentioning collisions with the walls
- claiming the number of molecules changes inside a sealed container
- giving the explanation for a volume change when the question asked about a temperature change
- calling pressure a force rather than a force acting over an area