A forensic scientist is investigating an ink sample taken from a suspected forged document. The scientist uses paper chromatography with two different solvents, Solvent A and Solvent B, to analyse the ink. In Solvent A, the ink produces a single spot. In Solvent B, the ink produces three separate spots. Explain what these results tell the scientist about the ink sample, and describe how the Rf value could be used to help identify one of the substances present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A forensic scientist uses paper chromatography to analyse an ink sample from a forged document. In Solvent A, the ink produces a single spot. In Solvent B, the ink produces three separate spots.
Model answer (5 marks)
1. The fact that Solvent B gives three distinct spots shows the ink contains more than one component – it is a mixture, not a single pure compound.
2. A pure compound would give only one spot in every solvent; the single spot seen in Solvent A does not prove purity.
3. The components separate because they differ in their solubility in the mobile phase and in their affinity for the stationary phase, giving different distribution ratios.
4. The Rf value is calculated as Rf = (distance travelled by the substance)/(distance travelled by the solvent front).
5. By comparing the Rf of a spot with the Rf values of known substances run under the same conditions, the scientist can identify that particular component of the ink.
2. A pure compound would give only one spot in every solvent; the single spot seen in Solvent A does not prove purity.
3. The components separate because they differ in their solubility in the mobile phase and in their affinity for the stationary phase, giving different distribution ratios.
4. The Rf value is calculated as Rf = (distance travelled by the substance)/(distance travelled by the solvent front).
5. By comparing the Rf of a spot with the Rf values of known substances run under the same conditions, the scientist can identify that particular component of the ink.
Examiner tips
- Use the command word ‘Explain’ – give reasons and link to theory. Show the Rf formula explicitly. Mention that comparison must be with the same solvent and paper. Keep each point brief and to the point.
Common mistakes
- Confusing the distance travelled by the solvent with the distance travelled by the spot. Assuming the single spot in Solvent A proves purity. Failing to state that comparison must be made with the same solvent system.
Mark scheme (5 marks)
- The result in Solvent B shows the ink is a mixture (not a pure substance) because it separates into more than one spot.
- A pure compound produces a single spot in all solvents, so the single spot in Solvent A alone does not confirm the ink is pure.
- The substances in the ink separate because they have different solubilities in the mobile phase / different distributions between the mobile phase and the stationary phase.
- The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent.
- The Rf value of each spot can be compared to known Rf values of substances in the same solvent to identify the compounds present in the ink.
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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