Explain why, at a given temperature, the particles in a sample of liquid water have a range of kinetic energies rather than all possessing the same kinetic energy.

IB DP Chemistry Higher Level (2023 syllabus) — S1.1 Introduction to the particulate nature 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 in constant, random motion and are constantly colliding with each other.
During these collisions kinetic energy is transferred between particles.
As a result, individual particles gain or lose kinetic energy at different times, producing a distribution of kinetic energies.
Temperature is a measure of the average kinetic energy of the particles, so whilst the average is fixed, individual values vary around this average.

Examiner tips

  • Use the word ‘random’ to show understanding of motion. Mention ‘collisions’ and ‘energy transfer’ to hit the key points. Explain that temperature represents the average, not the individual energies. Keep the answer concise – 4 points can be covered in 4 short sentences.

Common mistakes

  • Saying all particles have the same energy. Forgetting to mention collisions. Confusing temperature with kinetic energy of a single particle.

Mark scheme (4 marks)

  1. Particles in a liquid are in constant, random motion / constantly colliding with each other
  2. During collisions, kinetic energy is transferred between particles
  3. As a result, individual particles gain or lose kinetic energy at different times, producing a distribution of kinetic energies
  4. Temperature is a measure of the average kinetic energy of the particles, so whilst the average is fixed, individual values vary around this average

Key terms in this question

kinetic energy · temperature

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