A sports drink manufacturer produces a new energy drink. A food scientist wants to find out whether the orange colouring used in the drink is a pure substance or a mixture of dyes. She uses paper chromatography to investigate the colouring, using water as the solvent. After the experiment, the colouring produces three separate spots on the paper. Explain what this result tells the food scientist about the orange colouring, and describe the role of the mobile phase and stationary phase in causing the spots to separate.

Eduqas GCSE Chemistry — 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)

The orange colouring is a mixture, not a pure substance, because it gives three distinct spots on the paper.
A pure substance would give only one spot in any solvent.
The mobile phase (water) moves up the paper by capillary action and carries the dye molecules with it.
The dyes separate because they have different solubilities in the mobile phase and interact differently with the stationary phase (the cellulose paper), so they travel different distances.

Examiner tips

  • Use the word ‘mixture’ to show you understand the definition of a pure substance. Explain the role of the mobile phase first, then the stationary phase. Mention that different solubilities/affinities cause the varying distances travelled.

Common mistakes

  • Saying the mixture is ‘impure’ instead of ‘a mixture’. Forgetting to explain why the dyes travel different distances. Using vague terms like ‘different colours’ instead of ‘different solubilities/affinities’.

Mark scheme (4 marks)

  1. The orange colouring is a mixture (not a pure substance) because it produces more than one spot
  2. A pure substance would produce only one spot (in all solvents)
  3. The mobile phase (water/solvent) moves through the paper carrying the dyes with it
  4. The dyes separate because they have different solubilities in the mobile phase / interact differently with the stationary phase (paper), so they travel different distances up the paper

Key terms in this question

mobile phase · stationary phase · mixture · pure substance · paper chromatography

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