Use the kinetic theory of matter to explain what happens when a solid is heated until it melts and then continues to be heated until it becomes a gas.

WJEC GCSE Physics (Wales) — 1.8 Kinetic theory · 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 solid is heated the particles gain kinetic energy and vibrate more vigorously about their fixed positions.
At the melting point the particles have enough energy to break the forces that hold them in place, so they can move past one another and the solid becomes a liquid.
Continued heating gives the liquid particles more kinetic energy, so they move faster and more quickly.
At the boiling point the particles have enough energy to completely overcome the forces between them and escape, moving freely and independently in all directions – the substance becomes a gas.

Examiner tips

  • Use the phrase "gain kinetic energy" and "vibrate more vigorously" for the first point. Mention the melting point and the breaking of forces for the second. Show that heating increases particle speed for the third. Explain the boiling point as the energy overcoming all intermolecular forces for the fourth.

Common mistakes

  • Using vague terms like "heat up" instead of "gain kinetic energy". Forgetting to mention the breaking of forces at the melting point. Confusing the liquid state with the solid state in the explanation of melting.

Mark scheme (4 marks)

  1. When a solid is heated, the particles gain (kinetic) energy and vibrate more (vigorously) about their fixed positions
  2. At the melting point, particles have enough energy to break free from their fixed positions / overcome the forces holding them in place, so they can move/flow (liquid state)
  3. Continued heating gives the liquid particles more (kinetic) energy so they move faster / more quickly
  4. At the boiling point, particles have enough energy to completely overcome the forces between them and escape / move freely and independently in all directions (gas state)

Key terms in this question

kinetic theory

Related

More Kinetic theory questions

▶ Try answering this question with AI marking (free) →