A food technologist is investigating two fruit squash drinks, Sample A and Sample B. Both drinks appear identical in colour. The technologist carries out paper chromatography on each sample using the same solvent. Sample A produces three separate spots on the chromatogram. Sample B produces only one spot. The spot produced by Sample B has the same Rf value as one of the spots produced by Sample A. Explain what the results of this chromatography investigation tell the technologist about the composition of each sample, and explain why the Rf values are useful in identifying the substances present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food technologist is investigating two fruit squash drinks, Sample A and Sample B. Both drinks appear identical in colour. The technologist carries out paper chromatography on each sample using the same solvent. Sample A produces three separate spots. Sample B produces only one spot. The spot produced by Sample B has the same Rf value as one of the spots produced by Sample A.
Model answer (5 marks)
Sample A contains more than one substance – it gives three separate spots, so it is a mixture.
Sample B contains only one substance – it gives a single spot, so it is pure (in this solvent).
The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
Because different compounds have different Rf values in the same solvent, the Rf values can be used to identify which substances are present.
The fact that Sample B has the same Rf value as one of the spots in Sample A shows that the same compound is present in both samples.
Sample B contains only one substance – it gives a single spot, so it is pure (in this solvent).
The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
Because different compounds have different Rf values in the same solvent, the Rf values can be used to identify which substances are present.
The fact that Sample B has the same Rf value as one of the spots in Sample A shows that the same compound is present in both samples.
Examiner tips
- Use the word ‘mixture’ for multiple spots and ‘pure’ for one spot. State the Rf formula explicitly. Explain that identical Rf values mean identical compounds. Keep each point short and to the point.
Common mistakes
- Confusing Rf with retention factor. Saying ‘Sample B is a single compound’ without noting it’s pure in this solvent. Forgetting to mention that Rf values are solvent‑dependent.
Mark scheme (5 marks)
- Sample A is a mixture because it produces more than one spot / contains more than one substance
- Sample B is a pure substance because it produces only a single spot (in this solvent)
- The Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent
- Different compounds have different Rf values (in the same solvent), so Rf values can be used to identify the substances present
- Because Sample B has the same Rf value as one spot in Sample A, the same compound/substance is present in both samples
Key terms in this question
Rf value · paper chromatography
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