Explain why the temperature of a substance does not change while it is melting, even though energy is continuously being supplied to it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A block of a pure substance is placed in a heated container. Energy is supplied to the substance at a constant rate. The substance begins to melt, but a thermometer placed in the container shows no rise in temperature during this time.
Model answer (4 marks)
Energy supplied during melting is used to break the intermolecular forces between the molecules, so the energy goes into increasing the potential energy of the system rather than increasing the kinetic energy of the molecules.
Because the kinetic energy of the molecules does not increase, the average kinetic energy of the particles remains constant.
Temperature is a measure of the average kinetic energy of the particles, so if the kinetic energy does not change, the temperature remains constant.
Thus, although energy is continuously supplied, the temperature of the substance does not rise while it is melting.
Because the kinetic energy of the molecules does not increase, the average kinetic energy of the particles remains constant.
Temperature is a measure of the average kinetic energy of the particles, so if the kinetic energy does not change, the temperature remains constant.
Thus, although energy is continuously supplied, the temperature of the substance does not rise while it is melting.
Examiner tips
- Use the word "potential energy" to show you understand the energy change. Explain that temperature is linked to kinetic energy, not potential energy. Show the logical chain: energy supplied → breaks forces → no kinetic energy change → temperature constant.
- common_mistakes
- :
- Saying the temperature rises because energy is supplied. Confusing the energy used for breaking forces with kinetic energy. Forgetting to mention that the internal energy of the substance still increases.
Mark scheme (4 marks)
- Energy supplied goes towards separating / freeing the molecules from each other (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 internal energy of the substance still increases (potential energy of the molecules increases) during the change of state
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 →
More Change of state questions
- Evaporation from the surface of a liquid causes the liquid to cool. Explain why …
- Evaporation and boiling are both processes by which a liquid changes to a vapour…
- When candle wax is heated, it melts and becomes a liquid. When the liquid wax is…
- When a substance is heated at a constant rate, its temperature does not always r…