A sealed syringe contains a fixed amount of air. The plunger is pushed inward, reducing the volume of the air while the temperature stays the same. Explain, using kinetic theory, why the pressure of the air increases when its volume is decreased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A sealed syringe has its plunger pushed inward so the volume of the trapped air is made smaller. The temperature of the air does not change.
Model answer (5 marks)
Air consists of molecules that are in constant random motion.
Pressure is produced by the collisions of these molecules with the walls of the syringe.
When the plunger is pushed in, the volume available to the molecules is reduced, so they are confined to a smaller space.
With less space, the molecules travel shorter distances between collisions with the walls, so the collision frequency increases.
Higher collision frequency means a larger average force on the walls per unit area, which is an increase in pressure.
Pressure is produced by the collisions of these molecules with the walls of the syringe.
When the plunger is pushed in, the volume available to the molecules is reduced, so they are confined to a smaller space.
With less space, the molecules travel shorter distances between collisions with the walls, so the collision frequency increases.
Higher collision frequency means a larger average force on the walls per unit area, which is an increase in pressure.
Examiner tips
- Use the word "molecules" and "collisions" – the mark scheme rewards these terms.
- Show the chain: smaller volume → closer molecules → more frequent collisions → higher pressure.
- Keep the answer concise – 5 marks can be earned with 5 short points.
- Mention that temperature is constant so kinetic energy per molecule stays the same.
Common mistakes
- Saying the molecules move slower – they actually move at the same speed; only collision frequency changes.
- Confusing pressure with density – pressure rises because of collision frequency, not mass per volume.
- Using vague phrases like "more pressure" without linking to collisions or volume.
Mark scheme (5 marks)
- Air is made up of particles (molecules/atoms) that are constantly moving
- Pressure is caused by particles colliding with the walls of the container
- When volume decreases, the particles are closer together / confined to a smaller space
- Particles collide with the walls more frequently / more often
- More frequent collisions means a greater force on the walls per unit area, so pressure increases
Key terms in this question
kinetic theory · pressure · volume
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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