# A forensic scientist uses paper chromatography to identify an unknown ink found at a crime scene. The chromatography is carried out using ethanol as the solvent. The unknown ink produces three spots. The Rf values of the spots are 0.25, 0.51 and 0.78. Reference inks labelled P, Q and R each produce a single spot with Rf values of 0.51, 0.78 and 0.34 respectively. Explain what the results tell the scientist about the composition of the unknown ink, and explain why different compounds travel different distances up the chromatography paper.

> OCR A-Level Chemistry B: Salters (H433) — 6.3 Analysis (chromatography, NMR) · Explain · 5 marks

> A forensic scientist uses paper chromatography to separate the components of an unknown ink. Ethanol is used as the solvent. The unknown ink produces three separate spots with Rf values of 0.25, 0.51 and 0.78. Reference inks P, Q and R produce single spots with Rf values of 0.51, 0.78 and 0.34 respectively.

## Mark scheme (5 marks)

1. The unknown ink is a mixture (of at least three components / dyes / substances).
2. One component of the unknown ink matches reference ink P (Rf = 0.51).
3. One component of the unknown ink matches reference ink Q (Rf = 0.78).
4. The component with Rf = 0.25 does not match any reference ink (P, Q or R) / is unidentified.
5. Different compounds travel different distances because they have different attractions / interactions with the stationary phase (paper) and the mobile phase (solvent / ethanol); compounds more strongly attracted to the solvent travel further / compounds more strongly attracted to the paper travel less far.

## Key terms

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

## 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-forensic-scientist-uses-paper-chromatography-51d5c34f) · Published by Druglandscape Ltd.