# A chemist is investigating two white solid substances, substance A and substance B. Substance A melts at exactly 58 °C and substance B melts over the range 52 °C to 61 °C. Explain what the melting point data tells the chemist about the purity of each substance.

> WJEC A-Level Chemistry (Wales) — 1.6 Kinetics · Explain · 5 marks

## Mark scheme (5 marks)

1. Substance A is a pure substance
2. Because substance A has a sharp (single/specific) melting point
3. Substance B is a mixture / impure substance
4. Because substance B melts over a range of temperatures (rather than at a single fixed temperature)
5. Impurities lower the melting point and broaden the melting range / the different substances in the mixture melt at different temperatures causing the range

## Key terms

- [melting point data](https://www.gradenine.co.uk/glossary/melting-point-data)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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/a-chemist-is-investigating-two-white-fc6d5e27) · Published by Druglandscape Ltd.