A forensic scientist uses paper chromatography to analyse an ink sample taken from a document. The solvent travels 9.0 cm from the baseline. One of the substances in the ink travels 6.3 cm from the baseline. The scientist says the ink is a mixture, not a pure substance. Explain how paper chromatography works and how the results support the scientist's conclusion that the ink is a mixture.

WJEC A-Level Chemistry (Wales) — 1.7 Equilibria · Explain · 5 marks · View as Markdown

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

In paper chromatography, paper acts as the stationary phase and a solvent acts as the mobile phase. The solvent carries substances from the ink up the paper.

Model answer (5 marks)

Paper chromatography uses paper as the stationary phase and a solvent as the mobile phase. The solvent rises up the paper by capillary action and carries the components of the ink with it. Different components have different solubilities in the solvent and therefore move at different rates, giving separation.

A pure compound would give only one spot on the paper. In this experiment the ink produced a spot at 6.3 cm while the solvent front was at 9.0 cm. The Rf value is 6.3 ÷ 9.0 = 0.70, indicating that the ink contains at least two components that have moved to different positions. The presence of more than one spot shows that the ink is a mixture, not a single substance.

Examiner tips

  • Explain the roles of mobile and stationary phases first; then describe separation by differing solubilities. Show the Rf calculation and state its value. Mention that a single spot would indicate purity. Use the word ‘mixture’ to link the result to the conclusion.

Common mistakes

  • Calculating Rf incorrectly or omitting the calculation. Failing to explain why multiple spots mean a mixture. Using vague terms like ‘different substances’ without linking to solubility or Rf.

Mark scheme (5 marks)

  1. The mobile phase (solvent) moves through/up the stationary phase (paper), carrying the substances with it
  2. Substances separate because they have different solubilities in the mobile phase / travel at different speeds
  3. A pure compound would produce only a single spot in the solvent
  4. The ink produces more than one spot / the components separate into different spots, showing it is a mixture
  5. Correct calculation or use of Rf value: distance travelled by substance ÷ distance travelled by solvent = 6.3 ÷ 9.0 = 0.7

Key terms in this question

mixture · pure substance

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