A sealed container holds a fixed quantity of gas at constant temperature. Describe and explain what happens to the pressure of the gas when the volume of the container is reduced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The pressure of the gas increases when the volume is reduced.
This happens because the same number of gas particles are now confined to a smaller space, so they collide with the walls more frequently.
Each collision exerts a force on the wall; pressure is the perpendicular force per unit area.
Thus, with a smaller volume the force per unit area rises, giving a higher pressure.
This is consistent with the pressure–volume relationship: at constant temperature, pressure is inversely proportional to volume.
This happens because the same number of gas particles are now confined to a smaller space, so they collide with the walls more frequently.
Each collision exerts a force on the wall; pressure is the perpendicular force per unit area.
Thus, with a smaller volume the force per unit area rises, giving a higher pressure.
This is consistent with the pressure–volume relationship: at constant temperature, pressure is inversely proportional to volume.
Examiner tips
- Use the phrase "pressure increases" first. Show the link between reduced volume, more frequent collisions and higher force per area. Mention the definition of pressure. State the PV relationship to demonstrate consistency.
Common mistakes
- Saying the pressure stays the same or decreases. Forgetting to explain why collisions become more frequent. Using vague terms like "more particles" instead of the same number in a smaller volume.
Mark scheme (5 marks)
- The pressure of the gas increases (when volume is reduced)
- The gas particles collide with the walls of the container more frequently
- Because the same number of particles are now in a smaller volume / the particles are closer together
- Pressure is the perpendicular force per unit area on the walls of the container
- This is consistent with the pressure-volume relationship: pressure is inversely proportional to volume at constant temperature
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe and explain" question →
- Decode the mark scheme abbreviations →
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