A food colouring manufacturer claims that one of its red dyes is a pure substance. A technician carries out paper chromatography on the red dye using two different solvents. In both solvents, the red dye produces three separate spots on the paper. Explain what these results tell the technician about the red dye, and describe what result would be expected if the dye were a pure substance.

WJEC GCSE Chemistry (Wales) — 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)

The red dye is not a pure substance but a mixture. The appearance of three distinct spots in each solvent shows that three different components are present and have separated on the paper. If the dye were a pure compound, only one spot would be seen in each solvent. The fact that the same three‑spot pattern is obtained with two different solvents confirms that the dye is a mixture, not a single compound that would give a single spot in all solvents.

Examiner tips

  • Use the word ‘mixture’ to answer the first part; mention the three spots as evidence of multiple components.
  • State that a pure substance would give one spot in each solvent; contrast this with the observed result.
  • Show the result is consistent across solvents to reinforce the mixture conclusion.
  • Keep the answer concise – 4 marks are awarded for a short, clear explanation.

Common mistakes

  • Saying the dye is pure because it is red, ignoring the chromatography evidence.
  • Failing to mention that a single spot would be expected for a pure substance.
  • Not recognising that the same pattern in both solvents confirms the mixture, not just one solvent’s result.

Mark scheme (4 marks)

  1. The red dye is a mixture (not a pure substance)
  2. Three spots show that three different compounds are present / have separated
  3. A pure substance would produce only a single spot
  4. The result is the same in both solvents, which confirms the dye is a mixture (not a pure compound giving one spot in all solvents)

Key terms in this question

paper chromatography · pure substance

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