A student carries out paper chromatography on a food dye. The chromatogram produces three separate spots. Explain what this result tells the student about the food dye, and describe how the student could use Rf values to help identify the substances present.

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)

The chromatogram shows three distinct spots, so the food dye is not a single pure compound but a mixture of at least three different substances.

A pure compound would give only one spot in any solvent.

The Rf value of each spot is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front.

By comparing these Rf values with reference Rf values for known dyes (obtained using the same solvent system) the student can identify which substances are present in the mixture.

Examiner tips

  • State that three spots mean a mixture, not a pure compound. Explain the Rf calculation. Mention comparison with reference Rf values for identification.

Common mistakes

  • Saying the dye is pure because it has spots. Forgetting to divide by the solvent front distance. Comparing Rf values from different solvent systems.

Mark scheme (4 marks)

  1. The food dye is a mixture (not a pure substance) / contains three components / is not a single compound
  2. A pure compound would produce only one spot (in any solvent)
  3. The Rf value is calculated as the distance moved by the substance divided by the distance moved by the solvent
  4. The Rf values of the spots can be compared to known/reference Rf values (in the same solvent) to identify each substance

Key terms in this question

chromatography · Rf value

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