A food scientist is investigating the colour additives used in a range of sweets. She dissolves samples from five different sweets in water and runs a paper chromatography experiment using water as the solvent. After the experiment, she notices that some sweets contain a single dye whilst others contain a mixture of dyes. One spot produced by sweet D has an Rf value of 0.62. An approved food dye, Tartrazine, also produces a spot with an Rf value of 0.62 in the same experiment. Explain what Rf values are, and explain what can be concluded about the dye in sweet D compared to Tartrazine. Also explain why a dye that produces only one spot on the chromatogram is not necessarily a pure single substance.

OCR A-Level Chemistry B: Salters (H433) — CD Colour by design · Explain · 5 marks · View as Markdown

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

Paper chromatography is used in the food industry to identify colour additives in products. The Rf value of a substance is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. Different substances have characteristic Rf values under the same conditions.

Model answer (5 marks)

Rf value is the ratio of the distance travelled by the spot to the distance travelled by the solvent front.
Rf values are characteristic of a substance under the same conditions (same solvent, same paper).
The spot from sweet D has an Rf of 0.62, the same as Tartrazine, so the dye in sweet D could be Tartrazine.
However, matching Rf values alone do not prove identity; different substances can give the same Rf in one solvent, so further tests (e.g. a different solvent or other analytical methods) are required.
A single spot does not prove purity because two different substances that have the same Rf in that solvent would co‑move and appear as one spot.

Examiner tips

  • Define Rf clearly and state it is a ratio. Mention that Rf is characteristic under identical conditions. Explain the implication of matching Rf and note the need for further confirmation. Explain why one spot does not guarantee purity, citing co‑migration.
  • common_mistakes
  • :
  • Saying Rf is a percentage instead of a ratio. Claiming that identical Rf guarantees identity without mentioning further tests. Forgetting that a single spot can be a mixture of co‑moving dyes.

Mark scheme (5 marks)

  1. Rf value is the ratio of the distance moved by the substance (spot) to the distance moved by the solvent front
  2. Rf values are characteristic / unique to a substance under the same conditions (same solvent, same paper)
  3. The dye in sweet D has the same Rf value as Tartrazine, suggesting they could be the same substance
  4. Matching Rf values alone cannot confirm identity / further testing (e.g. using a different solvent) is needed because different substances can have the same Rf value in one solvent
  5. A single spot does not prove purity because two different substances with the same Rf value in that solvent would not be separated / would appear as one spot

Key terms in this question

Rf value · chromatography · solvent · dye

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