AQA Combined Science (8464) 6.3.3.1 · 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?
Combined 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.
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.
Key note · AQA 6.3.3.1 (8464)
Particle motion in gases
- Gas pressure: caused by molecular collisions with container walls.
- Higher T → faster molecules → more frequent, harder collisions → higher pressure.
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.