Explain, using particle theory, what happens to the particles in a liquid when it is heated until it becomes a gas.

Edexcel GCSE Chemistry (1CH0) — 2.1 States of matter · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Particles in a liquid are close together and move past each other.
Heating gives the particles more energy, so they move faster.
The increased energy allows particles to overcome the forces holding them together and escape from the liquid surface.
In the gas state, particles are spread far apart and move randomly in all directions.

Examiner tips

  • Use the phrase "particles move past each other" to show knowledge of liquid structure. Show the energy transfer by saying "more energy" or "move faster". Explain the escape of particles as overcoming intermolecular forces. Mention the final state as "spread far apart and move randomly" to cover all points.
  • common_mistakes
  • :
  • Omitting the movement of particles past each other in the liquid. Failing to state that particles escape the surface. Using vague terms like "become more energetic" without linking to faster movement.

Mark scheme (4 marks)

  1. Particles in a liquid are close together and move past each other / are randomly arranged and close together
  2. Heating gives the particles more energy / particles move faster
  3. Particles overcome the forces holding them together / escape from the liquid surface
  4. In the gas state, particles are spread far apart and move randomly in all directions

Key terms in this question

particle theory · liquid · gas

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