A student uses paper chromatography to investigate the dyes present in three different orange food colourings labelled P, Q and R. The student also runs a sample of a known pure dye called sunset yellow alongside the food colourings. After the experiment, the student measures the distances travelled by each spot and by the solvent front. Explain how the student can use the results of this chromatography experiment to determine whether sunset yellow is present in each of the three food colourings, and explain what it means if a food colouring produces only one spot on the chromatogram.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography can be used to identify the dyes present in food colourings. A reference substance is a known pure compound run alongside unknown samples so that comparisons can be made.
Model answer (5 marks)
1. Calculate the Rf value for each spot (distance travelled by spot ÷ distance travelled by solvent front). 2. Compare each Rf value from P, Q and R with the Rf value of the sunset yellow reference spot. 3. If a spot in any of the food colourings has the same Rf as sunset yellow, the student can conclude that sunset yellow is present in that colouring. 4. The solvent front moves dyes different distances because the dyes have different solubilities in the solvent and/or different affinities for the paper (stationary phase). 5. If a food colouring gives only one spot, it contains a single dye (i.e. it is not a mixture of dyes).
Examiner tips
- Show the Rf calculation for at least one spot. Mention that matching Rf values indicate the same compound. Explain the reason for different Rf values (solubility/affinity). State the implication of a single spot (pure dye).
Common mistakes
- Using the same Rf value for different dyes without justification. Confusing distance travelled by solvent front with distance travelled by spot. Failing to explain why a single spot indicates a pure dye.
Mark scheme (5 marks)
- The student calculates the Rf value for each spot (distance travelled by spot divided by distance travelled by solvent front)
- If the Rf value of a spot in P, Q or R matches the Rf value of the sunset yellow spot, then sunset yellow is present in that food colouring
- The solvent carries different dyes different distances because the dyes have different solubilities in the solvent (or different attractions to the paper/stationary phase)
- If a food colouring produces only one spot, it contains only one dye / it is a pure single dye (it is not a mixture of dyes)
- If a food colouring produces more than one spot, it is a mixture of dyes
Key terms in this question
chromatography · solvent front
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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