A student performs paper chromatography on two paint samples, Paint X and Paint Y, using the same solvent. Paint X produces one spot and Paint Y produces three spots. Explain what these results tell the student about the composition of each paint sample, and explain how the Rf value of a spot could be used to help identify one of the components.

AQA GCSE Chemistry (8462) — 4.8.1 Purity, formulations and chromatography · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Paint X shows only one spot, so it contains a single compound – it is pure or essentially a single component.
Paint Y shows three spots, indicating it is a mixture of three different compounds.
The Rf value of a spot is calculated as the distance the spot travels divided by the distance the solvent front travels.
By comparing a spot’s Rf value with reference Rf values for known substances (using the same solvent system) the student can identify that particular component.

Examiner tips

  • Use the command word ‘explain’ – give reasons and definitions. Show the Rf formula explicitly. Mention that the comparison must be in the same solvent system. Keep each point short and to the point.

Common mistakes

  • Saying the spot moves the same distance as the solvent front. Confusing Rf with the ratio of spot to solvent distance. Assuming a single spot means the paint is pure without noting it could be a single component mixture.

Mark scheme (4 marks)

  1. Paint X is a pure substance / contains only one compound / is not a mixture
  2. Paint Y is a mixture / contains three different compounds / components
  3. The Rf value is the distance moved by the substance divided by the distance moved by the solvent
  4. The Rf value can be compared to known / reference Rf values (in the same solvent) to identify the compound

Key terms in this question

Rf value · chromatography

Related

More Purity, formulations and chromatography questions

▶ Try answering this question with AI marking (free) →