A forensic scientist is investigating a suspected forgery. Two ink samples are taken — one from a genuine document and one from the suspected forgery. The scientist uses paper chromatography to compare the dyes in each ink sample. Explain how the results of the chromatography experiment could be used to determine whether the two ink samples contain the same dyes, and explain what it means if a spot has a high Rf value compared to a spot with a low Rf value.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography can be used to separate and identify the dyes present in an ink sample. Each dye travels a different distance up the chromatography paper depending on its solubility in the solvent and its attraction to the paper.
Model answer (5 marks)
1. Spots that appear at the same Rf value in both chromatograms show that the same dye is present in both ink samples.
2. If the chromatograms have a different number of spots, or spots at different Rf values, the inks contain different dyes and are therefore not the same.
3. Rf is calculated as the distance travelled by a spot divided by the distance travelled by the solvent front.
4. A high Rf value indicates the dye is more soluble in the solvent (or has a greater affinity for the solvent and a weaker attraction to the paper).
5. A low Rf value indicates the dye is less soluble in the solvent (or has a greater affinity for the paper and a stronger attraction to the stationary phase), so it travels a shorter distance.
2. If the chromatograms have a different number of spots, or spots at different Rf values, the inks contain different dyes and are therefore not the same.
3. Rf is calculated as the distance travelled by a spot divided by the distance travelled by the solvent front.
4. A high Rf value indicates the dye is more soluble in the solvent (or has a greater affinity for the solvent and a weaker attraction to the paper).
5. A low Rf value indicates the dye is less soluble in the solvent (or has a greater affinity for the paper and a stronger attraction to the stationary phase), so it travels a shorter distance.
Examiner tips
- Use the command word ‘explain’ – give a clear, step‑by‑step description of how matching Rf values prove sameness, and how differing Rf values prove difference. Show the Rf formula and give a brief interpretation of high vs low values. Use the exact wording from the mark scheme (e.g. ‘more soluble in the solvent’).
Common mistakes
- Confusing the distance travelled by the solvent front with the distance travelled by the spot. Assuming that a higher Rf always means a more powerful dye, rather than relating to solubility and affinity. Failing to mention that matching Rf values in both samples is the key evidence of sameness.
Mark scheme (5 marks)
- Spots at the same Rf value (or same distance from the baseline) in both samples indicate the same dye is present
- If the chromatograms show different numbers of spots or spots at different Rf values, the inks are different / do not contain the same dyes
- Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front
- A high Rf value means the dye is more soluble in the solvent (or has a greater affinity for the solvent / less attraction to the paper)
- A low Rf value means the dye is less soluble in the solvent (or has a greater affinity for the paper / stationary phase), so it does not travel as far
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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