A student puts a small amount of water into a syringe and then seals the nozzle. The student slowly pulls the plunger outwards, increasing the volume inside the syringe. The water turns into water vapour. Explain, using kinetic theory, why the water turns into water vapour when the volume is increased.

WJEC A-Level Physics (Wales) — 3.3 Kinetic theory · 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 water molecules are in constant random motion.
Increasing the volume gives the molecules more space, so the pressure inside the syringe falls.
At this lower pressure the boiling point of water is reduced, so the water can boil at a lower temperature.
The molecules in the liquid gain enough kinetic energy to overcome the attractive forces between them.
They escape from the liquid and move freely as a gas, i.e. water vapour.

Examiner tips

  • Use the exact phrase "boiling point is reduced" to hit the mark.
  • Show the causal chain: volume ↑ → pressure ↓ → boiling point ↓ → molecules escape.

Common mistakes

  • Confusing evaporation with boiling; the question is about boiling under reduced pressure.
  • Omitting the step that the pressure drop lowers the boiling point.

Mark scheme (5 marks)

  1. The water molecules are in constant random motion
  2. Increasing the volume increases the space available, so the pressure inside the syringe decreases
  3. At lower pressure, the boiling point of water is reduced / water can boil at a lower temperature
  4. The molecules in the liquid gain enough energy to overcome the forces of attraction between them
  5. Molecules escape from the liquid and move freely as a gas / vapour

Key terms in this question

kinetic theory · vapour

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