A fixed quantity of gas is trapped inside a cylinder fitted with a movable piston. The gas is compressed at constant temperature so that its volume is reduced. Explain what happens to the pressure of the gas as its volume decreases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A fixed quantity of gas is trapped inside a cylinder fitted with a movable piston. The piston is pushed inward slowly so that the temperature of the gas remains constant throughout.
Model answer (5 marks)
The quantity of gas is fixed, so the number of particles does not change. Reducing the volume confines the particles to a smaller space, so they collide with the walls more often. Each collision exerts a perpendicular force; more collisions per second give a larger total force on the walls. Hence the pressure increases, in line with the inverse pressure–volume relationship at constant temperature.
Examiner tips
- Use the word ‘fixed’ to show the number of particles is constant.
- Explain that smaller volume → higher collision frequency → higher force per unit area.
- Mention the inverse relationship between pressure and volume at constant temperature.
- Keep the answer concise and use the term ‘pressure’ correctly.
Common mistakes
- Failing to state that the number of particles is constant.
- Confusing temperature change with pressure change.
- Using vague terms like ‘more gas’ instead of ‘more collisions’.
Mark scheme (5 marks)
- The number of gas particles remains constant (fixed quantity of gas)
- Reducing the volume means the particles are confined to a smaller space
- Particles collide with the walls of the container more frequently
- Each collision exerts a perpendicular force on the walls, so more collisions per second means a greater total force on the walls
- Therefore the pressure increases, consistent with the pressure-volume relationship (pressure is inversely proportional to volume at constant temperature)
Key terms in this question
pressure · volume · fixed quantity of gas
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