A student is given two ink samples, ink A and ink B. The student uses paper chromatography to investigate whether the inks are pure substances or mixtures. Ink A produces a single spot and ink B produces three spots. Explain what these results show about the composition of each ink, and explain how the mobile phase and stationary phase are involved in separating the components of ink B.

AQA GCSE Chemistry (8462) — 4.8.3 Identification of ions by chemical and spectroscopic means (Chem only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Ink A is a pure substance because it gives only one spot in the chromatogram. Ink B is a mixture because it gives three distinct spots. In paper chromatography the mobile phase (solvent) moves up the paper by capillary action. The components of ink B are carried by the solvent but they travel different distances because they have different solubilities in the mobile phase and/or different interactions with the stationary phase. This differential movement separates the components into the three spots.

Examiner tips

  • State that a single spot indicates a pure substance; multiple spots indicate a mixture.
  • Explain that the solvent is the mobile phase and moves the ink components up the paper.
  • Mention that separation occurs due to differing solubilities and interactions with the stationary phase.
  • Use the exact terminology: mobile phase, stationary phase, solubility, interaction.

Mark scheme (4 marks)

  1. Ink A is a pure substance because it produces only one spot (in this solvent)
  2. Ink B is a mixture because it produces more than one spot / three spots
  3. The mobile phase (solvent) carries the components of ink B up the paper
  4. The components separate because they have different solubilities in the mobile phase / interact differently with the stationary phase, so they travel different distances

Key terms in this question

pure substance · mixture · mobile phase · stationary phase · chromatography

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