# A chemist has produced a sample of an unknown solid compound and wants to check whether it is pure. Explain how melting point data can be used to evaluate the purity of the solid, and describe what results the chemist would expect to see for both a pure substance and an impure substance.

> Edexcel GCSE Chemistry (1CH0) — 2.2 Methods of separating and purifying substances · Explain · 4 marks

## Mark scheme (4 marks)

1. A pure substance has a sharp/specific melting point (melts at one precise temperature)
2. An impure substance melts over a range of temperatures (not at a single point)
3. An impure substance has a lower melting point than the pure substance / impurities lower the melting point
4. The chemist can compare the measured melting point to a known/literature value to confirm the identity AND purity of the substance

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Chemistry (1CH0)](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-has-produced-a-sample-5862b69e) · Published by Druglandscape Ltd.