Explain how paper chromatography can be used to determine whether a coloured substance is pure or a mixture, and how the Rf value helps to identify a substance.

Eduqas A-Level Chemistry — 1.7 Equilibria · Explain · 5 marks · View as Markdown

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

A student carries out paper chromatography on two food dyes, labelled X and Y. Dye X produces a single spot and dye Y produces three spots after the solvent has moved up the paper.

Model answer (5 marks)

A pure substance will give a single spot on the paper, because all of the molecules travel the same distance. A mixture will give two or more spots, showing that more than one compound is present. The separation occurs because the different compounds have different solubilities in the mobile phase and therefore distribute differently between the mobile phase and the stationary phase. The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. By comparing the Rf value with literature values for known substances, the identity of the compound can be determined, and different compounds will have distinct Rf values in the same solvent system.

Examiner tips

  • Use the command word ‘explain’ – describe the process and give reasons.
  • Show the Rf formula and mention comparison to known values.
  • Include both purity test and identification aspects.
  • Keep sentences short and use exam‑specific terminology.

Common mistakes

  • Saying the spot moves with the solvent instead of explaining separation mechanism.
  • Confusing Rf with retention time or forgetting to divide by solvent distance.
  • Assuming all spots are impurities without recognising possible co‑elution.

Mark scheme (5 marks)

  1. A pure substance produces a single spot in paper chromatography
  2. A mixture produces more than one spot, showing it contains more than one compound
  3. Substances separate because they have different solubilities in the mobile phase / they are distributed differently 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 can be compared to known values to identify the substance / different compounds have different Rf values in the same solvent

Key terms in this question

paper chromatography · Rf value

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