Explain, in terms of particles, what happens to the internal energy of a substance when it melts, and why the temperature of the substance does not change during melting.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A solid substance is heated steadily until it has completely melted. During the melting process, the temperature of the substance remains constant even though energy is continuously being supplied.
Model answer (5 marks)
During melting, energy supplied to the substance is used to break bonds between particles, increasing their potential energy. The kinetic energy of the particles remains unchanged, so the average kinetic energy – and therefore the temperature – stays constant. Because internal energy is the sum of kinetic and potential energies, the internal energy rises while the temperature does not change.
Examiner tips
- Use the word ‘potential energy’ when describing bond breaking
- Explain that temperature reflects average kinetic energy
- Show that kinetic energy stays constant during melting
- State that internal energy increases due to added potential energy
Common mistakes
- Confusing temperature rise with energy supplied
- Saying the temperature rises during melting
- Forgetting to mention that kinetic energy does not change
Mark scheme (5 marks)
- During melting, energy is supplied to the particles (of the substance)
- The energy supplied increases the potential energy of the particles (as bonds/intermolecular forces are broken/overcome)
- The kinetic energy of the particles does not increase during melting
- Temperature is a measure of the average kinetic energy of the particles, so temperature does not change if kinetic energy does not change
- The internal energy increases during melting because internal energy is the sum of kinetic and potential energies of the particles, and potential energy has increased
Key terms in this question
internal energy · melting · temperature
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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