A student carries out paper chromatography on a sample of food colouring to find out whether it is a pure substance or a mixture. The solvent travels 8.0 cm up the paper. One spot is produced, and it travels 6.0 cm. Explain what the result tells the student about the food colouring, and describe how the Rf value of the spot could be used to help identify the substance present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography is an analytical technique used to separate and identify substances in a mixture. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.
Model answer (5 marks)
Only one spot is produced, so the food colouring is a pure substance (or contains only one compound/colour). A mixture would have produced more than one spot. The Rf value is calculated as 6.0 cm ÷ 8.0 cm = 0.75. This Rf value can be compared with known/reference Rf values for the same solvent system; if the value matches a reference, the substance can be identified. The comparison must use the same solvent, because different solvents give different Rf values for the same compound.
Examiner tips
- State that one spot = pure substance; more spots = mixture.
- Show the Rf calculation 6.0÷8.0=0.75.
- Explain that the Rf is compared to reference values using the same solvent.
Common mistakes
- Claiming that a single spot proves purity without mentioning the possibility of a single component mixture.
- Using the wrong solvent or not specifying the solvent when comparing Rf values.
- Failing to calculate the Rf value correctly or omitting units.
Mark scheme (5 marks)
- Only one spot is produced, so the food colouring is a pure substance (or contains only one compound/colour)
- A mixture would have produced more than one spot
- Correct calculation of the Rf value: 6.0 ÷ 8.0 = 0.75
- The Rf value can be compared to known/reference Rf values (for the same solvent) to identify the substance
- The comparison must be made using the same solvent, as different compounds have different Rf values in different solvents
Key terms in this question
paper chromatography · pure substance · 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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