A student carries out paper chromatography on two black inks, Ink A and Ink B, using the same solvent. Ink A produces a single spot and Ink B produces three separate spots. Explain what these results tell the student about the two inks, and describe the role of the mobile phase and the stationary phase in separating the components of Ink B.

WJEC GCSE Chemistry (Wales) — 1.4 The ever-changing Earth · 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, as it gives only one spot on the paper.
Ink B is a mixture, giving three separate spots, so it contains at least three different components.
In paper chromatography the mobile phase (the solvent) moves up the paper by capillary action.
The solvent carries the components of Ink B upward.
The components separate because they have different solubilities in the mobile phase; the more soluble components travel further, while the less soluble ones remain closer to the origin, giving distinct spots.

Examiner tips

  • State that a single spot indicates purity, 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 in the solvent.
  • Use the exact terms "mobile phase" and "stationary phase" as required.

Common mistakes

  • Claiming that a single spot means the ink is a single element rather than a pure compound.
  • Confusing the roles of mobile and stationary phases, e.g. saying the paper is the mobile phase.
  • Failing to explain that separation is due to different solubilities in the solvent.

Mark scheme (4 marks)

  1. Ink A is a pure substance because it produces only one spot
  2. Ink B is a mixture because it produces more than one spot / contains more than one substance
  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 / travel different distances depending on how soluble they are

Key terms in this question

mobile phase · stationary phase · paper chromatography

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