A sports drink manufacturer claims that the orange colouring used in their drink is a single pure substance. A student carries out paper chromatography on the orange colouring using two different solvents. In solvent A, the orange colouring produces three separate spots. In solvent B, the orange colouring produces a single spot. Explain what the results in each solvent tell us about the orange colouring, and state what the student should conclude overall about the manufacturer's claim.

WJEC A-Level Chemistry (Wales) — 1.2 Electronic structure · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

In solvent A the orange colouring gives three separate spots. This shows that the sample contains more than one substance – a mixture – because each component moves a different distance depending on its solubility in the mobile phase and its affinity for the stationary phase.

In solvent B the colouring gives a single spot. A pure compound would give a single spot in any solvent, but a single spot in one solvent does not prove that the sample is pure, because a mixture could also co‑move.

Therefore the evidence from solvent A is decisive: the orange colouring is a mixture. The manufacturer's claim that it is a single pure substance is incorrect.

The Rf values of the three spots in solvent A could be used to identify the individual components if reference standards are available.

Examiner tips

  • Show the logic: three spots = mixture; one spot alone not enough; conclude mixture; mention Rf values for identification

Common mistakes

  • Assuming the single spot in solvent B proves purity; ignoring that a mixture can co‑move; not explaining why different solubilities give different Rf values

Mark scheme (5 marks)

  1. The three spots in solvent A show that the orange colouring is a mixture (containing more than one substance/compound)
  2. The three substances travel different distances because they have different solubilities in the mobile phase (solvent A) / distribute differently between the mobile and stationary phases
  3. A pure compound produces a single spot in all solvents, so the single spot in solvent B alone does not prove purity
  4. The result from solvent A (three spots) is evidence that the colouring is a mixture, so the manufacturer's claim is incorrect / false
  5. The Rf values of the three spots (in solvent A) could be used to help identify the individual substances present in the mixture

Key terms in this question

pure substance · paper chromatography

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