Describe what happens to the particles in a substance when it changes from a liquid to a gas, and explain why substances with stronger forces between their particles have higher boiling points.

WJEC GCSE Chemistry (Wales) — 2.5 The production of important chemicals · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When a liquid is heated to its boiling point the particles gain kinetic energy.
The increased energy overcomes the attractive forces between the particles, so the forces are broken and the particles can separate.
They then move freely and independently, forming a gas.
Because stronger forces require more energy to break, a higher temperature is needed to supply that energy, so substances with stronger inter‑particle forces have higher boiling points.

Examiner tips

  • Use the word "gain kinetic energy" to show understanding of heating. Explain that forces are broken before particles can move freely. Show the logical link: stronger forces → more energy needed → higher boiling point.

Common mistakes

  • Saying particles simply "move faster" without mentioning energy or force breaking. Forgetting to state that the forces must be overcome before gas formation. Using vague terms like "more heat" instead of "more energy".

Mark scheme (4 marks)

  1. Particles in a liquid gain energy (when heated to the boiling point)
  2. The forces between the particles are overcome / broken, allowing particles to separate and move freely / independently
  3. Stronger forces between particles require more energy to overcome
  4. Therefore a higher temperature (boiling point) is needed to supply enough energy to overcome these stronger forces

Key terms in this question

boiling point

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