Explain why the temperature of a substance does not change whilst it is melting, even though it is still being heated.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student heats a solid substance. They notice that for a period of time the temperature remains constant, even though the substance is still receiving heat energy.
Model answer (4 marks)
The heat supplied increases the internal energy of the solid.
During the melting stage this energy is used to break or overcome the forces between the particles, i.e. to increase their potential energy.
Because the kinetic energy of the particles does not increase, the average kinetic energy remains constant.
Temperature is proportional to the average kinetic energy, so the temperature stays constant while the substance melts.
During the melting stage this energy is used to break or overcome the forces between the particles, i.e. to increase their potential energy.
Because the kinetic energy of the particles does not increase, the average kinetic energy remains constant.
Temperature is proportional to the average kinetic energy, so the temperature stays constant while the substance melts.
Examiner tips
- Use the term ‘latent heat of fusion’ to show you know the energy is used to break bonds.
- Explain that temperature is a measure of kinetic energy, not total internal energy.
- Show the sequence: heat added → potential energy ↑, kinetic energy unchanged → temperature unchanged.
- Mention that the process continues until all particles have changed state.
Mark scheme (4 marks)
- The energy supplied increases the internal energy of the substance
- During melting, the energy is used to break/overcome the bonds (forces) between particles / increase potential energy of particles
- The kinetic energy of the particles does not increase during melting
- Temperature is related to the average kinetic energy of the particles, so temperature does not rise
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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