A food manufacturer produces a fruit drink by mixing water, sugar, and several different food colourings together. A student carries out paper chromatography on the fruit drink to investigate the food colourings it contains. The student finds that one of the spots produced matches the Rf value of a known pure food colouring, whilst another spot separates into three separate spots. Explain what the chromatography results tell the student about the food colourings in the fruit drink.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The spot that matches the known Rf value shows that the corresponding food colouring is a pure substance – it behaves exactly like the reference sample. The Rf value is a characteristic of a compound, so any spot with the same Rf must be the same compound.
The spot that splits into three separate spots indicates that the colouring is not pure. It is a mixture of three different components, each of which has a different Rf value. The components separate because they have different affinities for the mobile and stationary phases (i.e. different solubilities in the solvent).
The spot that splits into three separate spots indicates that the colouring is not pure. It is a mixture of three different components, each of which has a different Rf value. The components separate because they have different affinities for the mobile and stationary phases (i.e. different solubilities in the solvent).
Examiner tips
- Use the word ‘pure’ for the matching spot and ‘mixture’ for the split spot. Mention that Rf is characteristic of a compound. Explain separation by differing solubilities/affinities.
- common_mistakes
- :
- Saying the split spot is a single compound. Forgetting that the same Rf means the same compound. Not linking the split to different solubilities or affinities.
Mark scheme (4 marks)
- The spot that matches the known Rf value indicates that colouring is a pure substance / single compound
- The Rf value can be used to identify the substance / substances with the same Rf value are the same compound
- The spot that separates into three spots shows that colouring is a mixture / not a pure substance
- The three components of the mixture separate because they have different Rf values / because they are differently distributed between the mobile phase and stationary phase / because they have different solubilities in the solvent
Key terms in this question
Rf value · paper chromatography
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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