A forensic scientist uses paper chromatography to compare an ink sample taken from a forged document with ink samples from three different pens, labelled P, Q and R. After running the chromatogram, the solvent front has travelled 8.0 cm. The ink from the forged document produces spots at 2.0 cm and 6.0 cm from the baseline. Pen P produces a single spot at 6.0 cm. Pen Q produces spots at 2.0 cm and 6.0 cm. Pen R produces spots at 2.0 cm, 4.5 cm and 6.0 cm. Explain how the scientist can use these results to identify which pen was used to write the forged document, and state the conclusion.

Cambridge International IGCSE Chemistry (0620) — 12.3 Chromatography · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The ink from the forged document gives two spots at 2.0 cm and 6.0 cm, so it contains two components. Pen Q also gives spots at exactly 2.0 cm and 6.0 cm, meaning its ink has the same two components as the forged ink. Pen P gives only a 6.0 cm spot (one component) and Pen R gives spots at 2.0 cm, 4.5 cm and 6.0 cm (three components). Therefore the ink in the forged document matches the ink from Pen Q. Conclusion: the forged document was written with Pen Q.

Examiner tips

  • Use the command word ‘explain’ – describe the reasoning, not just state the conclusion.
  • Show the comparison of spot positions clearly – 2.0 cm and 6.0 cm match.
  • Mention the number of components to rule out Pens P and R.
  • End with the conclusion that Pen Q was used.

Common mistakes

  • Confusing distance from baseline with distance travelled – always use distance from baseline.
  • Failing to note that Pen R has an extra spot at 4.5 cm, so it cannot match.
  • Writing the conclusion without linking it to the comparison of spots.

Mark scheme (4 marks)

  1. Spots that travel the same distance (from the baseline) in the same solvent are the same substance / component
  2. The forged document ink produces two spots / is a mixture of two components
  3. Pen Q produces spots at exactly the same distances (2.0 cm and 6.0 cm) as the forged document ink
  4. Conclusion: pen Q was used to write the forged document / pen Q's ink matches the forged document ink

Key terms in this question

paper chromatography · baseline · solvent front

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