A student carries out paper chromatography to investigate a food colouring. The solvent travels 8.0 cm up the paper. One of the spots travels 6.0 cm. The student claims the food colouring is a pure substance because only one spot is visible. Explain why the student's claim may be incorrect, and describe how Rf values can be used to help identify the substances present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food colouring is tested by paper chromatography. The solvent front travels 8.0 cm. A single spot is observed at 6.0 cm.
Model answer (5 marks)
A pure substance should give a single spot in every solvent used; one spot in a single solvent does not prove purity.
If two different compounds have the same Rf value in that solvent they will co‑move and appear as one spot, so a mixture can look pure.
The Rf value is calculated as Rf = distance travelled by the substance ÷ distance travelled by the solvent front.
Because different compounds have characteristic Rf values in a given solvent, the measured Rf can be compared with literature values to identify the substance.
A compound that is more soluble in the mobile phase will travel farther and therefore have a higher Rf value than a less soluble compound.
If two different compounds have the same Rf value in that solvent they will co‑move and appear as one spot, so a mixture can look pure.
The Rf value is calculated as Rf = distance travelled by the substance ÷ distance travelled by the solvent front.
Because different compounds have characteristic Rf values in a given solvent, the measured Rf can be compared with literature values to identify the substance.
A compound that is more soluble in the mobile phase will travel farther and therefore have a higher Rf value than a less soluble compound.
Examiner tips
- Show the definition of Rf and the calculation; include the formula. Explain that a single spot in one solvent is insufficient evidence of purity. Mention the possibility of overlapping spots with identical Rf values. Relate Rf to solubility in the mobile phase.
Common mistakes
- Assuming one spot automatically means a pure substance. Forgetting to divide by the solvent front distance. Not recognising that identical Rf values can mask a mixture.
Mark scheme (5 marks)
- A pure substance produces a single spot in all solvents, so one spot in one solvent is not sufficient evidence alone to confirm the substance is pure
- Two substances with the same Rf value in one solvent could overlap into a single spot, making a mixture appear to be a pure substance
- The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
- Different compounds have different Rf values in different solvents, so the Rf value can be compared to known values to help identify the substance
- The more soluble a substance is in the mobile phase (solvent), the further it travels and the higher its Rf value
Key terms in this question
Rf value · paper chromatography · pure substance
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