A food scientist tests two samples of a red food colouring. Sample A is a known pure compound. Sample B is a food colouring product bought from a shop. Both samples are analysed using paper chromatography with the same solvent. Sample A produces one spot and Sample B produces three spots. Explain what these results tell the scientist about the two samples, and describe the role of the mobile phase in separating the substances in Sample B.

WJEC GCSE Chemistry (Wales) — 1.2 Atomic structure and the periodic table · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Sample A is a pure substance, so it gives only one spot.
Sample B is a mixture, giving three spots.
The mobile phase (solvent) carries the substances up the paper.
In Sample B the different compounds travel different distances because they have different solubilities in the mobile phase, so they are separated.

Examiner tips

  • Use the word ‘pure’ for Sample A and ‘mixture’ for Sample B. Explain that the solvent is the mobile phase. Show that separation is due to differing solubilities in the solvent. Keep each point short and to the point.

Common mistakes

  • Saying Sample B is ‘impure’ instead of ‘a mixture’. Forgetting to mention the mobile phase as the solvent. Claiming separation is due to size rather than solubility.

Mark scheme (4 marks)

  1. Sample A is a pure substance / contains only one compound
  2. Sample B is a mixture / contains more than one compound
  3. The mobile phase (solvent) carries the substances / mixture up the paper
  4. Substances in Sample B travel different distances because they have different solubilities in the mobile phase / solvent

Key terms in this question

mobile phase · paper chromatography

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