# Explain what happens to the internal energy of a fixed mass of water as it is heated from 20 °C until it has completely turned to steam at 100 °C.

> AQA A-Level Physics (7408) — 3.6.2 Thermal Physics · Explain · 5 marks

## Mark scheme (5 marks)

1. Internal energy increases as water is heated from 20 °C to 100 °C
2. Because the kinetic energy of the molecules increases (as temperature rises)
3. During boiling/change of state, the temperature stays constant / does not change
4. During boiling, internal energy still increases / energy is still being transferred to the water
5. The potential energy of the molecules increases during boiling as intermolecular bonds/forces are broken / molecules are separated

## Key terms

- [internal energy](https://www.gradenine.co.uk/glossary/internal-energy)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-what-happens-to-the-internal-1a444098) · Published by Druglandscape Ltd.