A food scientist is testing a new fruit squash drink to find out how many different colourings it contains. She uses paper chromatography to separate the mixture of colourings. Explain how paper chromatography separates the colourings in the drink and how the scientist could use the results to determine whether any of the colourings is a pure substance.

Eduqas GCSE Chemistry — 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)

Paper chromatography separates the colourings by using a solvent (the mobile phase) that moves up the paper (the stationary phase). As the solvent rises it carries the different colourings with it. Colourings that are more soluble in the solvent travel further up the paper, while those less soluble move more slowly, so the mixture is separated into distinct spots. A pure colouring will give only one spot on the chromatogram. If a colouring produces more than one spot, it indicates that the colouring is a mixture and not a pure substance.

Examiner tips

  • Use the term ‘mobile phase’ for the solvent and ‘stationary phase’ for the paper.
  • Explain the movement of the solvent and the differential solubility of the colourings.
  • State that a single spot indicates purity, multiple spots indicate a mixture.
  • Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing the roles of mobile and stationary phases.
  • Failing to mention that solubility determines the distance travelled.
  • Assuming that a single spot always means purity without noting the possibility of co‑elution.

Mark scheme (4 marks)

  1. The solvent (mobile phase) moves through/up the paper (stationary phase), carrying the colourings with it.
  2. Colourings that are more soluble in the solvent travel further up the paper, separating the colourings from one another.
  3. A pure colouring will produce only one spot on the chromatogram.
  4. If a colouring produces more than one spot, it is a mixture / not a pure substance.

Key terms in this question

paper chromatography · pure substance · mixture

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