Revision notes for AQA GCSE Physics Particle motion in gases. Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.
Particle motion in gases
What you'll learn
- How gas molecules move inside a container.
- Why temperature is linked to the average kinetic energy of gas molecules.
- How gas pressure is caused by collisions with the walls.
- Why heating a gas at constant volume increases its pressure.
The particle model: gases are made of moving particles
The particle model is the idea that all substances are made from tiny particles. In a gas, these particles are usually called molecules in GCSE Physics, although some gases are made from single atoms.
Gas molecules are much further apart than particles in solids or liquids. They are not fixed in place, so they can move freely in all directions and spread out to fill the whole container.
Definition
Molecule
A molecule is a tiny particle made from two or more atoms joined together. In this topic, “gas molecules” means the tiny particles that make up the gas.
Constant random motion
The molecules of a gas are in constant random motion.
- Constant means they are moving all the time.
- Random means their directions and speeds are not all the same or organised.
- Motion means movement.
Gas molecules travel in straight lines between collisions, then change direction when they collide with other molecules or with the walls of the container.
Key Idea
Gas molecules never sit still
In a gas, molecules are moving all the time in random directions. This motion is the starting point for explaining both temperature and pressure.
Kinetic energy and temperature
Definition
Kinetic energy
Kinetic energy is the energy an object has because it is moving. Faster-moving particles have more kinetic energy.
The temperature of a gas is related to the average kinetic energy of its molecules.
This word average is important. At any moment, not every molecule has exactly the same speed. Some are moving faster, some slower. But if the temperature of the gas increases, the average kinetic energy of the molecules increases.
So, for a gas:
- higher temperature means greater average kinetic energy
- greater average kinetic energy means molecules are moving faster on average
- lower temperature means lower average kinetic energy
- lower average kinetic energy means molecules are moving more slowly on average
Common Mistake
Thinking every molecule has the same speed
Temperature tells you about the average kinetic energy of the molecules. It does not mean every molecule in the gas is moving at exactly the same speed.
Celsius and Kelvin
In everyday life, temperature is often measured in degrees Celsius, written °C. In physics, especially when comparing gas temperatures mathematically, we often use kelvin, written K.
For this section, you mainly need the qualitative idea: increasing temperature increases the average kinetic energy of the gas molecules.
Common Mistake
Using temperature relationships carefully
If you later use a direct proportionality involving gas temperature, the temperature must be in kelvin, not degrees Celsius. For this section, focus on the particle explanation rather than a formula.
What is gas pressure?
Definition
Pressure
Pressure is the force exerted per unit area. It tells you how concentrated a force is over a surface.
Pressure is measured in pascals, written Pa. One pascal means one newton per square metre.
For any surface:
p=FAp = \frac{F}{A}p=AFwhere:
- ppp is pressure in pascals, Pa
- FFF is force in newtons, N
- AAA is area in square metres, m²
In a gas, pressure is caused by molecules colliding with the walls of the container. Each collision gives the wall a tiny push. There are huge numbers of molecules, so all those tiny pushes add up to a measurable pressure.
Example
Calculating pressure on a wall
A gas exerts a force of 60 N on a wall with area 0.030 m². Calculate the pressure on the wall.
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Choose the pressure equation because you know force and area:
p=FAp = \frac{F}{A}p=AF -
Substitute the values, keeping the units with the quantities:
p=60 N0.030 m2p = \frac{60\ \text{N}}{0.030\ \text{m}^2}p=0.030 m260 N -
Calculate the pressure:
p=2000 Pap = 2000\ \text{Pa}p=2000 Pa
How moving molecules create pressure
When a gas molecule hits the wall of a container, it changes direction. Because its motion changes, it exerts a force on the wall during the collision.
A single molecule has a tiny effect. But in a real gas, there are enormous numbers of molecules hitting the walls again and again. The total effect of all these collisions is the pressure of the gas.
There are two key features of the collisions:
- how often molecules collide with the walls
- how hard the molecules collide with the walls
Faster molecules usually collide with the walls more often and with greater force during each collision.
The diagram shows the same gas in a rigid container at lower and higher temperature.
Key Idea
Pressure comes from collisions
Gas pressure is caused by molecules colliding with the walls of the container. Faster molecules cause more frequent and harder collisions, so the pressure increases.
Heating a gas at constant volume
Now put the ideas together.
A gas is held at constant volume when the size of its container stays the same. A sealed, rigid metal can is a good model: the gas cannot escape, and the container cannot expand significantly.
Definition
Constant volume
Constant volume means the space occupied by the gas stays the same. The container does not expand or shrink.
When you heat a gas at constant volume:
- energy is transferred to the gas by heating
- the temperature of the gas increases
- the average kinetic energy of the molecules increases
- the molecules move faster on average
- they collide with the container walls more often
- they collide with the walls harder
- the force on the walls increases
- the pressure increases
That is the full GCSE explanation.
Example
Explaining pressure increase in a sealed container
A sealed rigid container of gas is heated. Explain why the pressure of the gas increases.
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The container is rigid and sealed, so the gas stays at constant volume and the molecules cannot escape.
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Heating increases the temperature of the gas, so the average kinetic energy of the molecules increases.
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The molecules move faster on average, causing more frequent and harder collisions with the container walls.
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These collisions exert a greater total force on the walls. Since the wall area is unchanged, the pressure increases.
Cooling a gas at constant volume
The same chain works backwards.
If a gas at constant volume is cooled:
- its temperature decreases
- the average kinetic energy of the molecules decreases
- molecules move more slowly on average
- collisions with the walls become less frequent and less forceful
- the pressure decreases
This is why pressure changes in a sealed rigid container can be used as evidence that molecular motion has changed.
Tip
Use a cause-and-effect chain
For explanation questions, write the chain in order: temperature changes, average kinetic energy changes, molecular speed changes, collisions change, pressure changes.
Why “constant volume” matters
The phrase constant volume is not just decoration. It controls what happens to the gas.
If the gas is in a flexible container, like a balloon, heating it may make the balloon expand. In that case, the pressure may not increase in the same simple way because the gas has more space to spread into.
At GCSE for this sub-topic, the required explanation is specifically for a gas held at constant volume.
Common Mistake
Forgetting the fixed container
Do not just say “heating a gas increases pressure” without checking the condition. The clear GCSE statement is: heating a gas at constant volume increases its pressure.
A strong explanation sentence
A good exam answer might sound like this:
“Raising the temperature increases the average kinetic energy of the gas molecules. They move faster, so they collide with the container walls more frequently and with greater force. Since the volume is constant, this increases the pressure.”
That answer links the microscopic behaviour of molecules to the macroscopic measurement of pressure.
Exam technique
In the exam
- Use particle language: mention molecules, random motion, and collisions with the walls.
- For heating at constant volume, include the full chain: temperature increases, average kinetic energy increases, molecules move faster, collisions are more frequent and harder, pressure increases.
- Do not say “the molecules expand” or “heat rises” — the key idea is that the molecules move faster on average.
Self review
Check yourself
- Why does a gas exert pressure on the walls of its container?
- What happens to the average kinetic energy of gas molecules when the temperature increases?
- Why must the volume be constant when explaining that heating a gas increases its pressure?
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