A food scientist is investigating whether a sample of a red food colouring is a pure substance or a mixture. She carries out paper chromatography using two different solvents, solvent A and solvent B. In solvent A the sample produces a single spot. In solvent B the sample produces three separate spots. Explain what the results from both solvents tell the food scientist about the sample, and explain how paper chromatography separates the components of a mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food scientist uses paper chromatography to investigate a red food colouring sample. In solvent A, the sample produces a single spot. In solvent B, the same sample produces three separate spots.
Model answer (5 marks)
In solvent A the sample gives a single spot, but this alone does not prove the sample is pure, because a pure compound can give one spot in one solvent yet separate in another.
In solvent B the sample gives three distinct spots, which shows that the sample contains at least three different substances – i.e. it is a mixture.
Paper chromatography separates components because the paper acts as the stationary phase while the solvent is the mobile phase. The solvent carries the sample up the paper. Different components move at different speeds depending on their solubility in the solvent; the more soluble a component is in the mobile phase it travels further up the paper, giving separate spots.
In solvent B the sample gives three distinct spots, which shows that the sample contains at least three different substances – i.e. it is a mixture.
Paper chromatography separates components because the paper acts as the stationary phase while the solvent is the mobile phase. The solvent carries the sample up the paper. Different components move at different speeds depending on their solubility in the solvent; the more soluble a component is in the mobile phase it travels further up the paper, giving separate spots.
Examiner tips
- Mention that a single spot in one solvent is not conclusive for purity; contrast with multiple spots in another solvent.
- Explain the roles of stationary and mobile phases and link solubility to migration distance.
Common mistakes
- Claiming that a single spot in any solvent proves purity.
- Confusing the paper with the solvent as the mobile phase.
- Not explaining that separation is due to differing solubilities in the solvent.
Mark scheme (5 marks)
- The single spot in solvent A does not confirm the sample is pure on its own / a pure compound produces a single spot in all solvents
- The three spots in solvent B show the sample is a mixture (contains more than one compound/substance)
- In paper chromatography the paper is the stationary phase and the solvent is the mobile phase
- The solvent carries the substances up the paper / substances travel at different speeds through the paper
- Separation depends on how soluble each substance is in the solvent / the more soluble a substance is in the mobile phase, the further up the paper it travels
Key terms in this question
paper chromatography · mixture · pure substance
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