# A student is given a white powder and is told it is either a pure substance or a mixture. Explain how melting point data could be used to determine whether the white powder is a pure substance or a mixture, and describe what the student would expect to observe in each case.

> OCR A-Level Chemistry B: Salters (H433) — 2.2 Amount of substance · Explain · 5 marks

> A student is investigating an unknown white powder in a school laboratory. The student uses melting point data to evaluate the purity of the sample.

## Mark scheme (5 marks)

1. A pure substance has a sharp (precise/specific) melting point
2. If the powder is a pure substance, it will melt at one fixed/consistent temperature (no range)
3. A mixture melts over a range of temperatures (not at a single fixed point)
4. The melting point of a mixture is lower than expected for a pure substance (melting point depression) OR the presence of impurities/other substances lowers/broadens the melting point
5. The student could compare the measured melting point to a known/data book value to confirm identity and purity of the pure 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)
- [mixture](https://www.gradenine.co.uk/glossary/mixture)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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-student-is-given-a-white-6b6eb937) · Published by Druglandscape Ltd.