AQA Physics (8463) 4.3.3.1–4.3.3.3 · Model
Particle motion in gases
An aerosol can is thin metal holding a gas. How does a gas push on the walls of its container, and what changes when you heat it?
Triple Science · Higher tier
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“Do not burn, even after use.”
Every aerosol can carries that warning. An “empty” can is not empty: once the spray runs out, it is still sealed and still full of gas.
If you had to guess, when the can heats up on a fire, does it burst because the leftover gas catches fire inside it, or because the hot gas pushes harder on the walls?
A gas is mostly empty space. Its molecules are in constant random motion: each one travels in a straight line, in any direction, until it hits another molecule or a wall of the container, then sets off in a new direction. Every hit on a wall gives that part of the wall a tiny push. Huge numbers of hits every second add up to a steady push on every part of the wall: the pressure of the gas. The bench models a sealed can with sixteen molecules. Heat it, and call what changes before it happens.
The gas bench · stage 1 of 2
Heat the sealed can from 20 °C to 300 °C.
What happens to the molecules?
Mark it like an examiner
Which lines earn a mark?
The question: a sealed container of gas is heated; explain why the pressure of the gas increases. Each card is one line from a pupil’s answer.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
A gas can be compressed or expanded by changing the pressure on it. The pressure of a gas produces a net force at right angles to the wall of its container, and to any surface the gas touches. Here the right-hand wall is a piston, moved slowly enough for the gas to stay at the same temperature: its molecules keep the same average speed. Predict the gauge before the piston moves.
The piston bench
Pull the piston out slowly, until the volume has doubled.
What happens to the pressure?
Equations
p V = constant
pressure (Pa) × volume (m³) = constant, for a fixed mass of gas at constant temperature.
p₁ V₁ = p₂ V₂ · p₂ = p₁ V₁ ÷ V₂ · V₂ = p₁ V₁ ÷ p₂
The two pressures must be in the same unit, and so must the two volumes. Convert only when they arrive in different units.
1 kPa = 1000 Pa · 1 m³ = 1 000 000 cm³
Key fact
Gas pressure comes from molecules hitting the walls. Heat a gas at constant volume and its molecules move faster, so they hit the walls more often and harder: the pressure rises.
Command words in this lesson
- Describe
- Say what the molecules do: random directions, constant motion, a range of speeds. No reasons needed.
- Explain
- Link every step: temperature, kinetic energy, speed, hits on the walls, pressure.
- Predict
- Say what will happen, using a pattern in the data or the model.
- Calculate
- Write p₁V₁ = p₂V₂, substitute, rearrange, then give the answer with its unit.
Key note · AQA 4.3.3.1–4.3.3.3 (8463)
Particle motion in gases
- Gas pressure: caused by molecular collisions with container walls.
- Higher T → faster molecules → more frequent, harder collisions → higher pressure.
- Smaller V → more frequent collisions → higher pressure.
- Boyle's Law: pV = constant.
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure why a hotter gas pushes harder?
The benches are a model: sixteen molecules moving in two dimensions stand in for the huge number in a real gas, and the gauge is scaled so the starting gas reads 100 kPa. The piston is pulled out slowly enough for the gas to stay at 20 °C. In the fast push, no energy escapes from the gas, and its temperature rise is scaled to match air.