A food scientist is investigating whether a sample of red food colouring is pure or a mixture of dyes. She uses paper chromatography to analyse the sample, then compares her results with known reference dyes. Explain how paper chromatography separates a mixture of dyes and how the scientist can use the results to determine whether the food colouring is pure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food scientist is investigating a red food colouring that may contain more than one dye. She uses paper chromatography with a solvent to separate the components of the sample.
Model answer (4 marks)
Paper chromatography separates dyes by the movement of the solvent (mobile phase) up the paper (stationary phase). As the solvent rises it carries the dyes with it, but each dye distributes differently between the paper and the solvent. Dyes that have a higher affinity for the paper travel more slowly, while those that prefer the solvent move faster, giving different distances travelled.
If the food colouring is pure, only one spot will appear on the chromatogram. If it is a mixture, two or more spots will be seen.
The scientist can compare the positions (R_f values) of the spots from the sample with those of known reference dyes. Matching positions indicate the same dye(s) are present, allowing the scientist to identify whether the sample is pure or contains multiple dyes.
If the food colouring is pure, only one spot will appear on the chromatogram. If it is a mixture, two or more spots will be seen.
The scientist can compare the positions (R_f values) of the spots from the sample with those of known reference dyes. Matching positions indicate the same dye(s) are present, allowing the scientist to identify whether the sample is pure or contains multiple dyes.
Examiner tips
- Show the role of mobile and stationary phases and affinity differences; mention R_f values; state that a single spot = pure, multiple spots = mixture; compare with references to identify dyes
Common mistakes
- Confusing the roles of mobile and stationary phases; forgetting to mention R_f values; assuming any spot means impurity without reference comparison
Mark scheme (4 marks)
- The solvent (mobile phase) moves through the paper (stationary phase), carrying the dyes with it.
- Different dyes travel different distances because they have different distributions between the mobile phase and stationary phase / different affinities for the paper and solvent.
- If the food colouring is pure, only one spot will be produced on the chromatogram.
- The spots from the food colouring sample can be compared to reference dyes; if the positions match those of known dyes, the scientist can identify which dye(s) are present.
Key terms in this question
chromatography · mixture · paper chromatography
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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