A student carries out paper chromatography to investigate two food colourings, X and Y. Food colouring X produces a single spot. Food colouring Y produces three spots. The student measures the distances travelled by one of the spots in Y and by the solvent front. Using this information, explain what conclusions the student can draw about food colourings X and Y, and describe how the student could use the Rf value to help identify the substance that produced the spot measured in Y.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Food colouring X produces a single spot on the chromatogram. Food colouring Y produces three spots. The solvent travels 8.0 cm from the baseline. One spot in food colouring Y travels 4.0 cm from the baseline.
Model answer (5 marks)
Food colouring X gives a single spot, so it is a pure substance or contains only one compound.
Food colouring Y gives three spots, so it is a mixture of at least three different compounds.
The spots in Y separate because the compounds have different solubilities (or affinities) for the mobile phase and the stationary phase.
The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front.
By comparing the calculated Rf value with literature or data‑book Rf values for known substances, the student can identify which compound produced the measured spot in Y.
Food colouring Y gives three spots, so it is a mixture of at least three different compounds.
The spots in Y separate because the compounds have different solubilities (or affinities) for the mobile phase and the stationary phase.
The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front.
By comparing the calculated Rf value with literature or data‑book Rf values for known substances, the student can identify which compound produced the measured spot in Y.
Examiner tips
- Use the command word ‘explain’ – give reasons and link to theory. Show the Rf formula and mention the solvent front distance. Mention comparison with data‑book values for identification.
Common mistakes
- Saying the spot travelled 4.0 cm without relating it to the solvent front. Confusing Rf with the actual distance travelled. Assuming X is pure without recognising it could be a single‑compound mixture.
Mark scheme (5 marks)
- Food colouring X is a pure substance / contains only one compound because it produces a single spot
- Food colouring Y is a mixture because it produces more than one spot / separates into three spots
- The substances in Y separate because they have different solubilities in the mobile phase / solvent
- Rf value = distance travelled by the substance ÷ distance travelled by the solvent
- The calculated Rf value can be compared to known / data-book Rf values to identify the substance
Key terms in this question
Rf value · paper chromatography
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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