A food scientist is checking whether a red food colouring used in a new drink is a pure substance or a mixture. The scientist carries out paper chromatography using water as the solvent. After the experiment, the scientist observes that the red food colouring has produced three separate spots on the chromatography paper. Explain what this result tells the scientist about the red food colouring, and describe what result would be expected if the food colouring were a pure substance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The red food colouring is not a pure substance but a mixture. It contains more than one compound, so during paper chromatography the different compounds separate into distinct spots. The separation occurs because the compounds have different solubilities in the mobile phase (water). If the colouring were a pure substance, only a single spot would appear on the chromatography paper, regardless of the solvent used, indicating that only one compound is present.
Examiner tips
- Use the word "mixture" to claim the result, then explain separation by solubility differences. Mention the single spot for a pure substance. Keep the answer to 5 points.
Common mistakes
- Saying the colouring is a pure substance because of a single colour. Confusing the solvent with the stationary phase. Forgetting to mention that a pure substance gives one spot in any solvent.
Mark scheme (5 marks)
- The red food colouring is a mixture (not a pure substance)
- Because it contains more than one compound / the different compounds separate into different spots
- The substances separate because they have different solubilities in the mobile phase (water / solvent)
- A pure substance would produce only a single spot on the chromatography paper
- A pure substance produces a single spot in all solvents / the single spot indicates only one compound is present
Key terms in this question
pure substance · mixture · paper chromatography
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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