# A student measures the melting point of a sample of naphthalene and finds that it melts between 74 °C and 81 °C. The accepted melting point of pure naphthalene is 80 °C. Explain what the student's results suggest about the sample of naphthalene, and describe how melting point data can be used to distinguish a pure substance from a mixture.

> Eduqas A-Level Chemistry — 1.5 Energetics · Explain · 5 marks

## Mark scheme (5 marks)

1. The sample is not pure / is impure / is a mixture
2. The sample melts over a range of temperatures (rather than at a single/sharp temperature), which indicates it is not pure
3. A pure substance has a sharp / specific melting point (melts at one precise temperature)
4. A mixture melts over a range of temperatures
5. The melting point of the impure sample is lower than the accepted value for pure naphthalene, which is further evidence of impurity

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](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-measures-the-melting-point-17d7b64b) · Published by Druglandscape Ltd.