AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Particle model of matter › Particle model and pressure
Practise Particle motion in gases. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the random motion of gas molecules explains the temperature and pressure of a gas. You need to link the temperature of a gas to the average kinetic energy of its molecules, and explain in words (no calculations) why heating a gas in a fixed volume increases its pressure. Questions are mainly 2 to 4 mark explanations in contexts such as tyres, aerosol cans and sealed containers.
Key facts
Gas molecules are in constant random motion
Higher temperature = greater average kinetic energy of the molecules
Gas pressure is caused by molecules colliding with the walls and exerting a force on them
Constant volume: temperature up → pressure up
Hotter → molecules faster → collide with the walls more often and with more force
Cooler → molecules slower → fewer, weaker collisions with the walls → lower pressure
Notes
Gas molecules and temperature
The molecules of a gas are in constant random motion. They move in all directions, at a range of speeds, and keep colliding with each other and with the walls of their container.
The temperature of a gas is related to the average kinetic energy of its molecules.
Heating a gas raises its temperature, so its molecules move faster on average. Cooling a gas slows its molecules down.
How a gas exerts pressure
diagram
Gas molecules constantly collide with the walls of their container.
In each collision, a molecule exerts a force on the wall. Huge numbers of molecules collide every second, so together they produce a steady force on the wall.
The pressure of the gas is the force that these collisions produce on each unit of area of the wall.
Each collision pushes on the wall. Force per unit area = pressure.
Heating a gas at constant volume
diagram
If a gas is in a sealed container that cannot expand (constant volume), increasing its temperature increases its pressure.
Why: the molecules gain kinetic energy and move faster on average, so they collide with the walls more often (more collisions each second) and with more force in each collision. Both effects increase the pressure.
Same volume, higher temperature: faster molecules, higher pressure.
Cooling the gas does the opposite: the molecules move more slowly, so they hit the walls less often and with less force, and the pressure falls.
Examples: a sealed aerosol can may burst if it is heated; car tyre pressure rises after a long drive because the air inside warms up.
You only need to explain this relationship in words; you do not calculate it.
How to answer each type of question
Describe the motion of gas molecules
1 to 2 marksGrade 3
Use the key words: constant (never stops) and random (all directions).
If the question mentions temperature, link it to the average kinetic energy (speed) of the molecules.
Example. Describe the motion of the molecules in a gas. (2 marks)
Show the model answerHide the model answer
The molecules move all the time / constantly (1) in random directions (1).
Explain how a gas exerts a pressure
2 marksGrade 5
Say that the molecules collide with the walls (surface).
Say that each collision exerts a force, and the force on each unit area is the pressure.
Example. A balloon is filled with air. Explain how the air molecules exert a pressure on the inside of the balloon.
Show the model answerHide the model answer
The air molecules collide with the inside surface of the balloon (1). Each collision exerts a force on the surface; the total force on each unit area is the pressure (1).
Don’t lose marks
Saying the molecules get bigger or expand when heated. They move faster; their size does not change.
Giving only one effect. Say the molecules hit the walls more often AND with more force.
Saying the pressure rises because the molecules collide with each other more. Pressure comes from collisions with the walls.
Writing just 'more collisions'. Say 'more collisions per second' or 'collide more frequently'.
Saying the molecules have 'more energy' without saying which kind: say kinetic energy.
More tips
Memory tricks
Pressure chain: hotter → faster molecules → hit the walls more often AND harder → higher pressure.
Constant volume + hotter = higher pressure. That is why aerosol cans warn you not to heat them.
Temperature = average kinetic energy, so 'hotter gas' always means 'faster molecules on average'.
Exam technique
Use the key phrases: 'constant random motion', 'average kinetic energy', 'collide with the walls more frequently', 'with greater force'.
Check what is held constant: these explanations are for a gas whose volume does not change.
In 3-mark explanations, build a chain: temperature → kinetic energy (speed) → collisions with the walls (frequency and force) → pressure.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Describe the motion of gas moleculesThey are in constant random motion, moving in all directions.
Grade 4
Link gas temperature to molecules' kinetic energyThe higher the temperature, the greater the average kinetic energy of the molecules, so the faster they move on average.
Grade 5
Explain how a gas exerts pressureMolecules collide with the walls of the container and exert a force on them; the force on each unit area is the pressure.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A sealed metal can contains air. The can is heated. The volume of the can does not change. What happens to the pressure of the air inside the can?
It increases.
State what causes the pressure of a gas on the walls of its container.
Collisions of the gas particles with the walls.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Which statement describes the motion of the molecules in a gas? Tick (✓) one box.[1]
The molecules are not moving.
The molecules vibrate about fixed positions.
The molecules are in constant random motion.
The molecules all move in the same direction.
(b) Which quantity is related to the average kinetic energy of the molecules in a gas? Tick (✓) one box.[1]
Density
Mass
Temperature
Volume
(c) A sealed metal can contains air. The can is heated. The volume of the can does not change. What happens to the pressure of the air inside the can?[1]
(d) Aerosol cans contain gas under pressure. They carry the warning ‘Do not heat’. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: The molecules are in constant random motion.
(b)Answer: Temperature
(c)Answer: It increases.
it increases
(d)Answer: Heating would increase the pressure of the gas, so the can could burst.
heating increases the pressure of the gas inside the can, so the can could burst / explode
Question 2Medium5 marks
A sealed glass flask contains air. The flask is put in a water bath and heated from 20 °C to 60 °C. The volume of the flask does not change.
(a) Explain why the pressure of the air in the flask increases. Your answer should refer to the motion of the air molecules.[3]
(b) A second sealed flask of air at 20 °C is cooled to 5 °C. Describe what happens to the motion of the air molecules and to the pressure of the air in this flask.[2]
Show the answer and mark scheme
(a)Answer: The molecules gain kinetic energy and move faster, so they hit the walls more often and with more force, increasing the force per unit area.
the (average) kinetic energy of the molecules increases / the molecules move faster (on average)
the molecules collide with the walls of the flask more often
the molecules exert a greater force on the walls in each collision
so the force on each unit area of the walls / the pressure increases
(b)Answer: The molecules move more slowly on average, so the pressure of the air decreases.
the molecules move more slowly (on average) / have a lower average kinetic energy
the pressure decreases
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