A fixed mass of water vapour is cooled until it becomes ice. Describe what happens to the arrangement, movement and energy of the water particles as the vapour first condenses into liquid water and then freezes into ice.

WJEC A-Level Physics (Wales) — 2.3 Capacitance (A-Level only) · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

During condensation the water vapour particles, which were previously far apart and moving rapidly, are drawn together by increasing intermolecular forces and come into contact with one another.

In the liquid state the particles are still close but can slide past each other, so the liquid can flow.

As the liquid freezes the particles become arranged in a fixed, tightly packed lattice and can only vibrate about their positions.

At each stage the internal (kinetic) energy of the particles decreases as the temperature falls.

The stronger intermolecular forces that develop during condensation and freezing keep the particles in a more ordered arrangement and reduce their motion.

Examiner tips

  • Use the word ‘condensation’ and ‘freezing’ to show you are describing the two stages; link each stage to particle arrangement, movement and energy.
  • Mention the increase in intermolecular forces and the decrease in kinetic energy – these are key points the scheme rewards.
  • Show that in the liquid particles can flow, but in ice they are fixed – this distinction is worth a mark.
  • Keep the answer concise – 5 marks, so one or two sentences per point is enough.

Common mistakes

  • Confusing the order of condensation and freezing, or describing the liquid as still ‘moving freely’ like a gas.
  • Omitting the role of intermolecular forces or the decrease in kinetic energy.
  • Using vague terms such as ‘closer together’ without specifying that particles are now in contact or that they can flow in the liquid but not in the solid.

Mark scheme (5 marks)

  1. During condensation, particles come closer together / move from being spread apart to being in contact with each other
  2. In the liquid state, particles can flow / move over each other (rather than moving freely/randomly at high speed as in the gas)
  3. Intermolecular forces (between particles) increase / become stronger as the substance condenses from gas to liquid
  4. During freezing, particles become tightly packed / arranged in fixed positions and can only vibrate
  5. The internal energy / kinetic energy of the particles decreases as the substance is cooled (at both stages)

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