A student carries out paper chromatography on a food dye sample to find out whether it is a pure substance or a mixture. The solvent travels 8.0 cm up the paper. One spot produced by the food dye travels 6.0 cm and a second spot travels 2.0 cm. Explain what the results tell the student about the food dye, and state what would be observed if a pure substance were tested instead.

Eduqas A-Level Chemistry — 1.3 Chemical bonding · Explain · 5 marks · View as Markdown

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

Paper chromatography separates substances based on how soluble they are in the mobile phase. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.

Model answer (5 marks)

The food dye gives two distinct spots on the paper.
1. The first spot moves 6.0 cm, giving Rf = 6.0 ÷ 8.0 = 0.75.
2. The second spot moves 2.0 cm, giving Rf = 2.0 ÷ 8.0 = 0.25.
3. Two different Rf values mean the sample contains two different substances.
4. The different Rf values arise because the two substances have different solubilities in the mobile phase.
5. If the dye were a pure substance, only one spot would appear, giving a single Rf value in any solvent.

Examiner tips

  • Show the two Rf calculations to gain full marks for the quantitative part.
  • Explain that two spots mean a mixture – this is the key point the scheme rewards.
  • Mention that a pure substance gives only one spot – a concise statement is enough.
  • Use the exact wording ‘different solubilities in the mobile phase’ as the scheme expects.

Common mistakes

  • Calculating Rf incorrectly (e.g. 8.0÷6.0 instead of 6.0÷8.0).
  • Forgetting to state that a pure substance would give only one spot.
  • Using vague language such as ‘different compounds’ without linking to Rf values.

Mark scheme (5 marks)

  1. The food dye produces two spots, showing it is a mixture (not a pure substance) / contains two different substances
  2. Correct Rf value calculated for the first spot: 6.0 ÷ 8.0 = 0.75
  3. Correct Rf value calculated for the second spot: 2.0 ÷ 8.0 = 0.25
  4. The two substances have different Rf values because they have different solubilities in the mobile phase / solvent
  5. A pure substance would produce only a single spot in all solvents

Key terms in this question

paper chromatography · pure substance

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