A student investigates how the pressure of a fixed mass of gas changes when its volume is changed at constant temperature. They use a sealed syringe connected to a pressure sensor. The student pushes the plunger in to reduce the volume of the gas, recording the pressure at each volume. Describe what the student should do to make this a fair test and explain the relationship between pressure and volume that the results should show.

OCR A-Level Physics A (H556) — 1.1 Practical skills assessed · Describe and Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student compresses a fixed mass of air inside a sealed syringe and measures the pressure at different volumes, keeping the temperature constant throughout.

Model answer (5 marks)

1. Keep the temperature constant by conducting the experiment in a temperature‑controlled environment or by allowing the syringe to equilibrate before measurements.
2. Ensure the syringe remains sealed so the amount of gas does not change.
3. The pressure–volume relationship follows Boyle’s law: P∝1/V.
4. When the plunger is pushed in, the gas volume decreases.
5. A smaller volume means gas molecules collide with the walls more often, increasing the force per unit area and therefore the pressure.

Examiner tips

  • Use the exact phrase "Boyle’s law" and state the proportionality P∝1/V to gain full marks.
  • Show the causal chain: ↓V → ↑collision frequency → ↑force per area → ↑pressure.
  • Mention the need for a temperature‑controlled setup and a sealed syringe to keep the test fair.

Common mistakes

  • Confusing the relationship as P∝V instead of P∝1/V.
  • Omitting the requirement to keep temperature constant or the syringe sealed.
  • Using vague language like "pressure changes" without explaining why.

Mark scheme (5 marks)

  1. The temperature must be kept constant throughout the investigation
  2. The mass (or amount) of gas must remain fixed / the syringe must remain sealed so no gas escapes
  3. Pressure and volume are inversely proportional
  4. As the plunger is pushed in, the volume decreases so gas particles collide with the walls of the syringe more frequently
  5. More frequent collisions with the walls result in a greater force per unit area, so pressure increases

Key terms in this question

pressure · volume

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