# A forensic scientist uses paper chromatography to separate the inks from two pens found at a crime scene. Ink A produces three spots with Rf values of 0.23, 0.51 and 0.78. Ink B produces three spots with Rf values of 0.51, 0.65 and 0.78. Explain what Rf values are and what the results tell the scientist about the two inks.

> Eduqas A-Level Chemistry — 3.7 Spectroscopy and chromatography · Explain · 5 marks

> Rf value = distance travelled by substance ÷ distance travelled by solvent

## Mark scheme (5 marks)

1. An Rf value is the ratio of the distance travelled by a substance (spot/dye/component) to the distance travelled by the solvent front
2. Rf values are fixed/characteristic for a given substance under the same conditions, so they can be used to identify substances
3. The two inks share two components in common (Rf values 0.51 and 0.78 appear in both)
4. The two inks are not identical / they are different inks because Ink A has a component with Rf 0.23 that Ink B does not have, and Ink B has a component with Rf 0.65 that Ink A does not have
5. The two pens / inks did not come from the same source / are not from the same pen, because their compositions are different

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-uses-paper-chromatography-324cb669) · Published by Druglandscape Ltd.