# A pharmacist is checking the purity of a white powder that should be a single painkiller compound. She measures the melting point of the powder and finds it melts over a range of temperatures between 148 °C and 156 °C. A reference source states that the pure painkiller compound has a sharp melting point of 153 °C. Explain what the melting point data tells the pharmacist about the purity of the powder, and describe one separation technique she could use to obtain a purer sample of the solid compound from its solution.

> WJEC A-Level Chemistry (Wales) — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks

> A pharmacist tests a white powder that should contain only one painkiller compound. The melting point of the powder is found to be between 148 °C and 156 °C. The pure compound has a known sharp melting point of 153 °C.

## Mark scheme (5 marks)

1. A pure substance has a sharp melting point (melts at one specific temperature)
2. The powder melts over a range of temperatures, so it is not a pure substance / it is a mixture
3. The powder therefore contains impurities / other substances mixed with the painkiller compound
4. Crystallisation should be named as the separation technique
5. Correct description of crystallisation: heat the solution until crystals start to form / until saturated, allow to cool so crystals form, then filter to collect the crystals

## 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-pharmacist-is-checking-the-purity-0eca4acf) · Published by Druglandscape Ltd.