A student carries out paper chromatography to investigate two food colourings, X and Y. Food colouring X produces three separate spots on the paper. Food colouring Y produces only one spot. The spot from food colouring Y appears at the same position as one of the spots from food colouring X. Explain what these results tell the student about the two food colourings, and describe how the Rf value of the single spot from food colouring Y could be used to help identify the substance it contains.

Eduqas A-Level Chemistry — 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)

Food colouring X is a mixture, because it gives three separate spots on the paper. Food colouring Y is a pure substance, because it gives only one spot in this solvent system. The single spot from Y has the same Rf value as one of the spots from X, so Y contains the same compound as that component of X.

The Rf value of the Y spot is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. By comparing this Rf value with reference Rf values for known substances (in the same solvent system) the student can identify the compound present in Y.

Examiner tips

  • Use the word ‘because’ to link reasoning to evidence; state each point clearly.
  • Show the Rf formula explicitly and mention comparison with reference values.
  • Keep the answer concise – 5 marks only.
  • Use UK spelling (e.g. ‘colouring’).

Common mistakes

  • Confusing Rf with retention factor (k).
  • Assuming Y is a mixture because it has one spot.

Mark scheme (5 marks)

  1. Food colouring X is a mixture (because it produces more than one spot)
  2. Food colouring Y is a pure substance (because it produces only one spot in this solvent)
  3. Food colouring Y contains the same substance as one of the components of food colouring X (because they appear at the same position / same Rf value)
  4. Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
  5. The Rf value can be compared to known / reference Rf values (in the same solvent) to identify the substance

Key terms in this question

paper chromatography · Rf value

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