A food scientist is investigating whether a sample of a yellow food colouring is a pure substance or a mixture. The scientist uses paper chromatography, with ethanol as the solvent, and obtains three separate spots on the paper. Explain what the results tell the scientist about the food colouring, and describe how the Rf value of one of the spots could be used to help identify the substance responsible for that spot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food scientist carries out paper chromatography on a yellow food colouring using ethanol as the solvent. The chromatogram produces three separate spots at different positions on the paper. The solvent travels 12.0 cm from the baseline. One of the spots travels 9.0 cm from the baseline.
Model answer (5 marks)
The appearance of three distinct spots shows that the yellow food colouring is not a pure substance but a mixture of at least three different components.
In paper chromatography the components are separated because they have different solubilities in the mobile phase (ethanol). The more soluble component travels further with the solvent front, while the less soluble component remains closer to the baseline.
The Rf value of a spot is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
For the spot that travelled 9.0 cm: Rf = 9.0 cm ÷ 12.0 cm = 0.75.
This Rf value can be compared with reference Rf values of known substances run under the same conditions (same paper, same solvent, same temperature). If the Rf matches that of a known pigment, the scientist can identify that component of the mixture.
In paper chromatography the components are separated because they have different solubilities in the mobile phase (ethanol). The more soluble component travels further with the solvent front, while the less soluble component remains closer to the baseline.
The Rf value of a spot is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
For the spot that travelled 9.0 cm: Rf = 9.0 cm ÷ 12.0 cm = 0.75.
This Rf value can be compared with reference Rf values of known substances run under the same conditions (same paper, same solvent, same temperature). If the Rf matches that of a known pigment, the scientist can identify that component of the mixture.
Examiner tips
- Show the link between multiple spots and mixture; explain solubility difference; state Rf formula; calculate 0.75; mention comparison to reference values for identification
Common mistakes
- Calculating Rf incorrectly (e.g. using 12 cm ÷ 9 cm); forgetting to explain why multiple spots mean a mixture; not stating the role of solubility in separation
Mark scheme (5 marks)
- Three spots show the food colouring is a mixture (not a pure substance)
- The three substances in the mixture have different solubilities in the mobile phase (ethanol), causing them to separate
- The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
- Correct Rf value calculated for the spot that travels 9.0 cm: Rf = 9.0 ÷ 12.0 = 0.75
- The Rf value can be compared to known/reference Rf values of substances in the same solvent to identify the substance
Key terms in this question
paper chromatography · Rf value · mixture · pure substance
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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