# A forensic scientist is testing an unknown white powder found at a crime scene. She dissolves a small amount of the powder in water and carries out paper chromatography using two different solvents. In solvent A, the sample produces a single spot. In solvent B, the sample also produces a single spot, but the spot appears at a different position on the paper. She also measures the melting point of the dry powder and finds it melts sharply at 134 °C. A reference database lists compound X as having a melting point of 134 °C. Explain what the results of both tests tell the scientist about the identity and purity of the unknown powder.

> WJEC A-Level Chemistry (Wales) — 1.3 Chemical bonding · Explain · 5 marks

## Mark scheme (5 marks)

1. A pure substance produces a single spot in chromatography (in all solvents), so the single spots in both solvents indicate the powder is a pure substance / not a mixture.
2. The Rf value (position of the spot) differs between the two solvents, which is expected because different compounds have different Rf values in different solvents.
3. The Rf value(s) can be compared to reference values / the database to help identify the substance.
4. A pure substance has a sharp melting point, so the sharp melting point at 134 °C indicates the powder is pure.
5. The melting point of 134 °C matches the database value for compound X, so the powder is likely to be compound X.

## Key terms

- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-is-testing-an-f9e52fc2) · Published by Druglandscape Ltd.