A quality control technician is investigating two samples of a yellow food colouring. Sample P produces a single spot when analysed by paper chromatography in three different solvents. Sample Q produces three separate spots in the same solvents. Explain what these results tell the technician about Sample P and Sample Q, and describe how paper chromatography separates the components in Sample Q.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography is used in the food industry to check whether colourings contain a single substance or a mixture of substances.
Model answer (5 marks)
Sample P is a pure substance – it contains only one compound or element. A pure compound gives a single spot in all solvents, so the single spot confirms its purity.
Sample Q is a mixture – it contains more than one compound. In paper chromatography the solvent (mobile phase) carries the components up the paper (stationary phase). The components separate because they have different solubilities in the solvent; the more soluble a substance is, the further it travels up the paper, giving separate spots.
Sample Q is a mixture – it contains more than one compound. In paper chromatography the solvent (mobile phase) carries the components up the paper (stationary phase). The components separate because they have different solubilities in the solvent; the more soluble a substance is, the further it travels up the paper, giving separate spots.
Examiner tips
- State that a single spot indicates purity; multiple spots indicate a mixture. Explain that the mobile phase carries components up the paper and that separation is due to differing solubilities.
- Use the exact terms ‘pure substance’, ‘mixture’, ‘mobile phase’, ‘stationary phase’, ‘solubility’ – these are the words the scheme rewards.
Common mistakes
- Claiming that multiple spots mean the sample is impure without recognising it is a mixture. Confusing the roles of mobile and stationary phases. Forgetting to mention solubility as the reason for separation.
Mark scheme (5 marks)
- Sample P is a pure substance / contains only one compound or element
- A pure compound produces a single spot in all solvents (so one spot confirms purity)
- Sample Q is a mixture / contains more than one compound
- In paper chromatography the solvent (mobile phase) carries the components up the paper (stationary phase)
- Components separate because they have different solubilities in the solvent / the more soluble a substance is, the further up the paper it travels
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Chemical bonding questions
- A student carries out paper chromatography on a food dye sample to find out whet…
- A food technologist is developing a new sports drink. The drink contains water, …
- A pharmacist is checking the purity of two batches of a white crystalline medici…
- A scientist is testing two samples of a soluble white solid. Sample A is known t…