A food colouring manufacturer claims that one of its yellow dyes is a pure substance. A chemist carries out paper chromatography on the yellow dye using two different solvents. In solvent A, the yellow dye produces a single spot. In solvent B, the yellow dye produces three separate spots. Explain what the results from both solvents tell the chemist about the yellow dye.

Eduqas GCSE Chemistry — 1.4 The ever-changing Earth · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A single spot in solvent A does not prove the dye is pure, because a pure compound can sometimes give a single spot in one solvent but not in another.

In solvent B the dye gives three distinct spots, showing that it contains at least three different components.

A pure compound would give only one spot in every solvent used.

Hence the yellow dye is a mixture, not a pure substance, and the manufacturer's claim is incorrect.

Examiner tips

  • State that one spot in one solvent is insufficient evidence of purity.
  • Explain that multiple spots in another solvent prove a mixture.
  • Mention that a pure compound gives a single spot in all solvents.
  • Conclude that the dye is not pure.

Common mistakes

  • Claiming that a single spot in any solvent proves purity.
  • Forgetting to explain the significance of multiple spots.
  • Using vague language like "it looks impure" instead of stating the mixture evidence.

Mark scheme (4 marks)

  1. A single spot in one solvent does not confirm purity on its own
  2. The three spots in solvent B show the dye is a mixture / not a pure substance
  3. A pure compound produces a single spot in all solvents
  4. Therefore the yellow dye is not a pure substance / the manufacturer's claim is incorrect

Key terms in this question

paper chromatography · pure substance

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