# A forensic scientist is investigating an ink sample taken from a suspected forged document. The scientist uses paper chromatography with two different solvents, Solvent A and Solvent B, to analyse the ink. In Solvent A, the ink produces a single spot. In Solvent B, the ink produces three separate spots. Explain what these results tell the scientist about the ink sample, and describe how the Rf value could be used to help identify one of the substances present.

> WJEC A-Level Chemistry (Wales) — 1.2 Electronic structure · Explain · 5 marks

> A forensic scientist uses paper chromatography to analyse an ink sample from a forged document. In Solvent A, the ink produces a single spot. In Solvent B, the ink produces three separate spots.

## Mark scheme (5 marks)

1. The result in Solvent B shows the ink is a mixture (not a pure substance) because it separates into more than one spot.
2. A pure compound produces a single spot in all solvents, so the single spot in Solvent A alone does not confirm the ink is pure.
3. The substances in the ink separate because they have different solubilities in the mobile phase / different distributions between the mobile phase and the stationary phase.
4. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent.
5. The Rf value of each spot can be compared to known Rf values of substances in the same solvent to identify the compounds present in the ink.

## Key terms

- [chromatography](https://www.gradenine.co.uk/glossary/chromatography)
- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)

## 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-forensic-scientist-is-investigating-an-2056fef0) · Published by Druglandscape Ltd.