A sealed syringe contains a fixed mass of gas at room temperature. The plunger is slowly pushed inward, reducing the volume of the gas. Explain why the pressure of the gas increases as the volume decreases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student pushes the plunger of a sealed syringe inward at a slow, steady rate so that the temperature of the gas inside remains constant.
Model answer (4 marks)
The gas molecules are in constant random motion and collide with the walls of the syringe.
Reducing the volume gives the molecules less space, so they are closer together.
The molecules therefore collide with the walls more frequently, giving more collisions per unit area per second.
The greater number of collisions per unit area produces a greater force per unit area on the walls, so the pressure increases.
Reducing the volume gives the molecules less space, so they are closer together.
The molecules therefore collide with the walls more frequently, giving more collisions per unit area per second.
The greater number of collisions per unit area produces a greater force per unit area on the walls, so the pressure increases.
Examiner tips
- Use the word ‘collide’ to show understanding of kinetic theory.
- Show the chain: less volume → closer molecules → more collisions → higher pressure.
- Mention that temperature is constant, so kinetic energy stays the same.
- Keep the answer concise and use UK spelling.
Common mistakes
- Failing to mention that collisions are more frequent, not just more force.
- Using ‘speed up’ instead of ‘more collisions per unit area’. Not stating that temperature remains constant, which could confuse the explanation.
Mark scheme (4 marks)
- The gas molecules are in constant random motion and collide with the walls of the syringe
- Reducing the volume means the molecules have less space / the molecules are closer together
- The molecules collide with the walls more frequently / there are more collisions per unit area per second
- The greater number of collisions per unit area means a greater force per unit area acts on the walls, so pressure increases
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Particle model and pressure (Phys only) questions
- Explain why the pressure inside a sealed gas container increases when the volume…
- A sealed metal canister contains a fixed mass of gas. The canister is placed in …
- A weather balloon is filled with a fixed mass of gas at ground level. As the bal…
- A bicycle tyre is inflated by pumping air into it. The volume of the tyre remain…