What causes gas pressure, and how does it change with temperature and volume?
Gas pressure in terms of particle collisions, and how pressure changes with temperature and with volume.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on gases, covering how gas pressure is caused by particle collisions, and how the pressure of a gas changes with temperature and with volume.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC Double Award Unit 6 wants you to explain gas pressure in terms of particle collisions, and how the pressure of a gas changes with temperature and with volume.
What causes gas pressure
Because the particles move in all directions, they hit all the walls, so the pressure acts in all directions. The more frequent and forceful the collisions, the higher the pressure.
Pressure and temperature
Cooling the gas does the opposite, lowering the pressure. This is why a sealed container of gas can burst if heated too much.
Pressure and volume
Linking to collision theory
The way gas pressure depends on how often and how hard the particles collide is the same idea as collision theory for reaction rates: both depend on the frequency and energy of collisions. A higher temperature means faster particles and more energetic, more frequent collisions, which raises both the gas pressure and the rate of a reaction. Spotting this connection between gas pressure and rates helps you explain both using the particle model.
Everyday examples of gas pressure
Gas pressure ideas explain many everyday things. A bicycle tyre gets harder when you pump more air in, because more particles in the same space collide with the walls more often. A balloon expands in the warm and shrinks in the cold as the pressure of the gas inside changes with temperature. Aerosol cans carry a warning not to heat them, because heating raises the pressure of the gas inside until the can could burst. Being able to apply the particle model to a familiar example like these shows real understanding and is often asked for.
Pressure acts in all directions
A key point is that gas pressure acts equally in all directions, not just downwards. This is because the gas particles move randomly in all directions, so they hit every wall of the container with the same frequency and force. This is why an inflated balloon is pushed out evenly into a round shape, and why a gas fills its container completely. Remembering that the random motion of the particles makes the pressure act in all directions helps explain the shapes that gases and inflated objects take.
Try this
Q1. What causes the pressure of a gas? [1 mark]
- Cue. The gas particles colliding with the walls of the container.
Q2. What happens to the pressure of a sealed gas if it is cooled? [1 mark]
- Cue. It decreases (the particles move slower and collide less often and less hard).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain, using the particle model, why the pressure of a gas in a sealed container increases when it is heated.Show worked answer →
A Unit 6 explain question worth 4 marks. Reward: heating the gas gives the particles more kinetic energy, so they move faster (1); they collide with the walls of the container more often (1) and with more force each time (1); since pressure is caused by these collisions, the pressure increases (1). Markers credit faster particles, more frequent collisions and harder collisions, leading to higher pressure. A common error is to say the particles get bigger.
WJEC style3 marksExplain, using the particle model, why squeezing a gas into a smaller volume increases its pressure.Show worked answer →
A Unit 6 explain question. Reward: reducing the volume means the same number of particles are in a smaller space (1); so they hit the walls more often (more frequent collisions per unit area) (1); since pressure is caused by collisions with the walls, the pressure increases (1). Markers credit the same particles in a smaller space, more frequent collisions and higher pressure. A common error is to say the particles move faster (that is the effect of temperature, not volume).
Related dot points
- The particle model of solids, liquids and gases, density, and changes of state as physical changes.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on the particle model, covering the arrangement of particles in solids, liquids and gases, density, and changes of state as physical changes.
- Specific heat capacity and the equation for energy, internal energy, and the energy needed for a change of state.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on heating, covering specific heat capacity and its equation, internal energy, and the energy needed during a change of state when the temperature stays constant.
- Work done, power, kinetic and gravitational potential energy, and the conservation of energy in mechanical situations.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on work and energy, covering work done, power, kinetic and gravitational potential energy, and the conservation of energy in mechanical situations.
- The rate of reaction as how fast reactants are used or products form, and collision theory in terms of frequency and energy of collisions.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on reaction rates, covering what the rate of reaction means and how collision theory explains rate in terms of the frequency and energy of collisions.