A student carries out paper chromatography on two food colourings, sample A and sample B, using the same solvent. Sample A produces three spots and sample B produces one spot. Explain what these results tell the student about each sample, and describe how the student could use Rf values to help identify the substances present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography is used to separate and identify substances in a mixture. The stationary phase is the paper and the mobile phase is the solvent.
Model answer (5 marks)
Sample A is a mixture, because it produces three spots on the chromatogram. Sample B is a pure substance, because it produces only one spot.
The Rf value of a spot is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
By comparing the Rf value of each spot with the Rf values of known reference substances run in the same solvent, the student can identify which compounds are present in sample A. Different compounds have different Rf values in a given solvent, so each spot in sample A can be matched to a specific food colouring.
The Rf value of a spot is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
By comparing the Rf value of each spot with the Rf values of known reference substances run in the same solvent, the student can identify which compounds are present in sample A. Different compounds have different Rf values in a given solvent, so each spot in sample A can be matched to a specific food colouring.
Examiner tips
- State that more than one spot = mixture, one spot = pure. Define Rf and show the formula. Explain comparison with reference Rf values. Mention that different compounds give different Rf values.
Common mistakes
- Confusing the solvent front distance with the spot distance. Assuming all spots are the same compound. Not recognising that a single spot indicates purity.
Mark scheme (5 marks)
- Sample A is a mixture (because it produces more than one spot / three spots)
- Sample B is a pure substance (because it produces only one spot)
- The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
- The Rf value of each spot can be compared to known/reference Rf values (of known substances in the same solvent) to identify the substances
- Different compounds have different Rf values (in a given solvent), so each spot in sample A can be identified separately
Key terms in this question
paper chromatography · Rf value
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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