A chemist is investigating whether a sample of red food colouring is a pure substance or a mixture. She carries out paper chromatography on the sample using water as the solvent. After the experiment, she observes three separate spots on the paper. Explain what these results tell the chemist about the food colouring, and explain how paper chromatography produces this separation.

WJEC GCSE Chemistry (Wales) — 1.1 The nature of substances and chemical reactions · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Three spots show the food colouring is a mixture, not a pure substance, because a pure substance would give only one spot. The solvent (mobile phase) moves up the paper by capillary action, carrying the different components of the mixture with it. The components separate because they have different solubilities in the solvent; the more soluble substances travel further up the paper, while the less soluble ones remain lower, producing distinct spots.

Examiner tips

  • State that three spots mean a mixture, one spot would mean pure.
  • Explain that the solvent moves up the paper and carries the mixture.
  • Mention that separation is due to differing solubilities (mobile phase affinity).
  • Use the exact wording from the mark scheme.

Mark scheme (4 marks)

  1. Three spots show the food colouring is a mixture (not a pure substance) / contains three different substances/compounds
  2. A pure substance would produce only one spot
  3. The solvent (mobile phase) moves up the paper carrying the substances in the mixture with it
  4. The substances separate because they have different solubilities in the solvent / travel different distances because more soluble substances travel further up the paper

Key terms in this question

paper chromatography · pure substance · mixture

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