A paint manufacturer produces a green paint by mixing together two pigments, one blue and one yellow. A student carries out paper chromatography on the green paint using a single solvent. After the experiment, only one spot is visible on the chromatography paper. Explain what this result tells the student about the green paint, and describe what the student would expect to see if the green paint were a mixture of the two pigments.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A paint manufacturer produces a green paint by mixing together two pigments, one blue and one yellow. A student carries out paper chromatography on the green paint using a single solvent. After the experiment, only one spot is visible on the chromatography paper.
Model answer (4 marks)
A single spot indicates the green paint behaves as a pure substance, meaning it contains only one compound.
This suggests the two pigments are not present as separate substances; the paint is not a mixture of two pigments.
If the green paint were a mixture of the two pigments, two separate spots would be visible on the paper.
The spots would appear at different heights/positions because the pigments have different solubilities in the mobile phase/solvent.
This suggests the two pigments are not present as separate substances; the paint is not a mixture of two pigments.
If the green paint were a mixture of the two pigments, two separate spots would be visible on the paper.
The spots would appear at different heights/positions because the pigments have different solubilities in the mobile phase/solvent.
Examiner tips
- Use the word ‘indicates’ to link the observation to the conclusion. Mention that a single spot means the sample is homogeneous. Explain that two spots would mean a heterogeneous mixture. Show the link between solubility and spot position.
Common mistakes
- Claiming the paint is pure without linking to the single spot. Confusing ‘pure substance’ with ‘single compound’. Forgetting to explain that different solubilities give different spot heights.
Mark scheme (4 marks)
- A single spot indicates the green paint behaves as a pure substance / contains only one compound
- This suggests the two pigments are not present as separate substances / the paint is not a mixture of two pigments
- If it were a mixture of two pigments, two separate spots would be visible on the paper
- The spots would appear at different heights / positions because the pigments have different solubilities in the mobile phase / solvent
Key terms in this question
paper chromatography · mixture
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