A fixed amount of gas is trapped inside a sealed bicycle pump with the nozzle blocked. A cyclist pushes the handle of the pump inward, compressing the gas into a smaller volume. The temperature of the gas does not change. Explain, using kinetic theory, why the pressure of the gas increases when its volume is decreased at constant temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a gas is compressed into a smaller volume at constant temperature, its pressure increases. This relationship can be explained using the kinetic theory of gases.
Model answer (5 marks)
The gas consists of many fast‑moving particles that collide with the walls of the pump.
Pressure is produced by the force of these collisions on the walls.
When the pump handle is pushed in, the volume available to the particles is reduced.
With a smaller volume the particles are confined to a tighter space and therefore collide with the walls more often – the collision frequency increases.
Because the temperature is unchanged, the average speed (and kinetic energy) of the particles remains the same, so the only change is the higher collision frequency, which gives a greater force on the walls and thus a higher pressure.
Pressure is produced by the force of these collisions on the walls.
When the pump handle is pushed in, the volume available to the particles is reduced.
With a smaller volume the particles are confined to a tighter space and therefore collide with the walls more often – the collision frequency increases.
Because the temperature is unchanged, the average speed (and kinetic energy) of the particles remains the same, so the only change is the higher collision frequency, which gives a greater force on the walls and thus a higher pressure.
Examiner tips
- Use the term ‘collision frequency’ to link volume change to pressure.
- Show that temperature constancy means kinetic energy stays constant, so only frequency changes.
- Mention that pressure is force per unit area, so more frequent impacts raise pressure.
Common mistakes
- Confusing temperature change with volume change – students may say the particles move faster, which is incorrect here.
- Failing to mention that the kinetic energy (speed) of the particles remains constant because temperature is constant.
- Using vague language like ‘more particles’ instead of ‘more collisions per second’.
Mark scheme (5 marks)
- The gas is made up of moving particles (molecules/atoms) that collide with the walls of the container.
- Pressure is caused by the force of these collisions on the walls.
- When the volume decreases, the particles are closer together / confined to a smaller space.
- The particles collide with the walls more frequently (more collisions per second).
- Because temperature is unchanged, the speed/kinetic energy of the particles stays the same, so the increased collision frequency alone causes a greater force on the walls and therefore higher pressure.
Key terms in this question
kinetic theory · pressure · volume · temperature
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Kinetic theory and gases (A-Level only) questions
- A gas is trapped inside a rigid steel cylinder. The cylinder is placed in a free…
- A driver is travelling along a motorway at a constant velocity when they suddenl…
- A car is travelling at high speed along a motorway when the driver notices a haz…
- A sealed metal canister contains gas at room temperature. The canister is placed…