When a substance is heated at a constant rate, its temperature does not always rise continuously. Explain why the temperature of a substance remains constant during a change of state, even though energy is still being supplied.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A block of ice is heated at a constant rate. As it melts, a thermometer placed in the mixture shows a steady reading, even though the heater continues to supply energy.
Model answer (4 marks)
When a substance is heated, the supplied energy is used to break the intermolecular forces that hold the particles together. This energy increases the potential energy of the particles rather than their kinetic energy. Because temperature is a measure of the average kinetic energy, the temperature remains constant while the phase change continues. The internal energy of the system rises, but the kinetic energy – and therefore the temperature – does not.
Examiner tips
- Use the phrase ‘breaks intermolecular forces’ to show understanding of energy use.
- Explain that temperature is linked to kinetic energy, not potential energy.
- Show that the internal energy increases but kinetic energy stays the same.
- Keep the answer concise – 4 points are enough for full marks.
Common mistakes
- Confusing temperature change with internal energy change.
- Saying the energy goes to ‘heat the particles’ instead of ‘break bonds’.
- Not linking temperature to kinetic energy explicitly.
Mark scheme (4 marks)
- Energy supplied goes towards separating the particles / breaking the intermolecular forces between molecules
- The energy supplied does not increase the kinetic energy of the molecules
- Temperature is a measure of the average kinetic energy of the particles, so if kinetic energy does not increase, temperature does not rise
- The energy supplied increases the potential energy of the molecules / the internal energy of the system increases due to an increase in potential energy
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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