# Explain why the temperature of a pure substance does not change during a change of state, even though thermal energy is continuously supplied to it.

> IB DP Physics Higher Level (2023 syllabus) — B.1 Thermal energy transfers · Explain · 4 marks

## Mark scheme (4 marks)

1. During a change of state, the supplied energy is used to overcome the intermolecular (potential) forces/bonds between molecules.
2. The average kinetic energy of the molecules therefore does not increase during the change of state.
3. Temperature is a measure of the average (random translational) kinetic energy of the molecules.
4. Since the average kinetic energy remains constant, the temperature remains constant (despite continued energy input).

## Key terms

- [change of state](https://www.gradenine.co.uk/glossary/change-of-state)
- [temperature](https://www.gradenine.co.uk/glossary/temperature)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-temperature-of-a-d18abfc0) · Published by Druglandscape Ltd.