# A technician is investigating three paint samples — paint P, paint Q and paint R. He carries out paper chromatography on each sample using the same solvent. Paint P produces a single spot that travels 4.8 cm, whilst the solvent front travels 6.0 cm. Paint Q produces three separate spots. Paint R produces a single spot that travels 3.6 cm, whilst the solvent front also travels 6.0 cm. The technician wants to determine whether any of the paints are pure substances and to identify what paints P and R might be by comparing their Rf values with a reference table of known pigments. Explain how the technician can use the chromatography results to determine which paints are pure substances, and calculate the Rf values of the spots in paints P and R, stating what these Rf values tell him about the identity of these paints.

> WJEC A-Level Chemistry (Wales) — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks

> Reference Rf values (solvent used: ethanol)
> • Pigment A: Rf = 0.80
> • Pigment B: Rf = 0.60
> • Pigment C: Rf = 0.45
> • Pigment D: Rf = 0.30

## Mark scheme (5 marks)

1. A pure substance produces a single spot in chromatography
2. Paints P and R are pure substances because each produces only one spot; paint Q is a mixture because it produces more than one spot
3. Rf value for paint P = 4.8 ÷ 6.0 = 0.80
4. Rf value for paint R = 3.6 ÷ 6.0 = 0.60
5. Paint P matches Pigment A (Rf = 0.80) and paint R matches Pigment B (Rf = 0.60), so the Rf values can be used to identify the pigments in P and R

## Key terms

- [pure substance](https://www.gradenine.co.uk/glossary/pure-substance)
- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)
- [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-technician-is-investigating-three-paint-0a1033fa) · Published by Druglandscape Ltd.