A student carries out paper chromatography on a food dye. After the experiment, the solvent has travelled 8.0 cm. The food dye produces only one spot, which has travelled 6.0 cm. Explain what the result tells us about the purity of the food dye, and describe how the Rf value could be used to help identify the substance.
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. A pure compound produces a single spot in all solvents, whereas a mixture separates into different spots.
Model answer (4 marks)
The single spot indicates the food dye is pure – only one compound is present.
The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent.
In this case Rf = 6.0 cm ÷ 8.0 cm = 0.75.
The Rf value can be compared with reference Rf values for known substances run in the same solvent system to identify the dye.
The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent.
In this case Rf = 6.0 cm ÷ 8.0 cm = 0.75.
The Rf value can be compared with reference Rf values for known substances run in the same solvent system to identify the dye.
Examiner tips
- State that a single spot means purity (1 mark).
- Define Rf and give the calculation (1 mark).
- Mention the calculated Rf value (1 mark).
- Explain comparison with reference values for identification (1 mark).
Common mistakes
- Saying the dye is pure because it has one spot without noting it means only one compound is present.
- Confusing Rf with the distance travelled by the solvent.
- Using the wrong solvent distance or spot distance in the calculation.
Mark scheme (4 marks)
- The food dye produces only one spot, so it is a pure substance (contains only one compound/element, not mixed with any other substance)
- The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
- The Rf value for this dye is 0.75
- The Rf value can be compared to known/reference Rf values (in the same solvent) to identify the substance
Key terms in this question
paper chromatography · Rf value
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