A student carries out paper chromatography on a food colouring and obtains three separate spots on the paper. Explain what this result tells the student about the food colouring, 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).
A food manufacturer claims that one of its food colourings is a pure substance. A student dissolves a sample of the food colouring in water and carries out paper chromatography using water as the solvent.
Model answer (5 marks)
Three spots mean the food colouring is a mixture, not a pure substance – a pure compound would give only one spot.
The student can calculate the Rf value for each spot:
Rf = (distance travelled by the spot)/(distance travelled by the solvent front).
By comparing each Rf value with reference Rf values for known dyes (obtained under the same conditions), the student can identify which substances are present in the mixture.
The student can calculate the Rf value for each spot:
Rf = (distance travelled by the spot)/(distance travelled by the solvent front).
By comparing each Rf value with reference Rf values for known dyes (obtained under the same conditions), the student can identify which substances are present in the mixture.
Examiner tips
- State that three spots indicate a mixture; a single spot indicates purity.
- Show the Rf formula and calculate each value.
- Mention that Rf values are compared with reference values in the same solvent system to identify components.
Common mistakes
- Failing to recognise that multiple spots mean a mixture.
- Using the wrong formula for Rf (e.g., dividing solvent distance by spot distance).
- Not comparing the calculated Rf values with reference data to identify the dyes.
Mark scheme (5 marks)
- Three spots means the food colouring is a mixture / is not a pure substance
- A pure compound would produce only one/a single spot
- Rf value is calculated as distance travelled by the substance divided by the distance travelled by the solvent
- Calculate the Rf value for each of the three spots
- Compare the Rf values obtained to known/reference Rf values (in the same solvent) to identify the substances
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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