A sealed plastic syringe contains a fixed mass of air. The open end of the syringe is blocked and the plunger is pushed inward so that the volume of the air decreases. The temperature of the air stays constant throughout. Explain what happens to the pressure of the air inside the syringe as the volume decreases, in terms of the molecules of the gas.

Eduqas A-Level Physics — 4.1 Thermal physics · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The gas molecules move randomly and are in continuous random motion.
Pressure is caused by the molecules colliding with the walls of the syringe.
When the volume decreases, the same number of molecules occupy a smaller space.
Thus the molecules collide with the walls more frequently.
More frequent collisions increase the force on the walls, so the pressure rises.

Examiner tips

  • Use the word "pressure" and link it to "collisions". Show the chain: smaller volume → more collisions → higher force → higher pressure. Mention that temperature is constant, so kinetic energy per molecule stays the same.

Common mistakes

  • Forgetting to state that the temperature is constant. Using vague terms like "more gas" instead of "more collisions". Omitting the link between collision frequency and pressure.

Mark scheme (5 marks)

  1. The gas molecules move randomly / are in continuous random motion
  2. Pressure is caused by molecules colliding with / hitting the walls of the syringe
  3. When volume decreases, the same number of molecules are in a smaller space
  4. Molecules collide with the walls more frequently / more often
  5. More frequent collisions mean the force on the walls increases, so pressure increases

Key terms in this question

pressure · volume · molecules

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