A manufacturer produces two versions of a paint: one described as a 'pure pigment' and one described as a 'formulation'. Explain how paper chromatography could be used to distinguish between the pure pigment and the formulation, and describe what the results would show for each sample.

WJEC A-Level Chemistry (Wales) — 1.2 Electronic structure · Explain · 5 marks · View as Markdown

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

A manufacturer produces two versions of a paint: one described as a 'pure pigment' and one described as a 'formulation'. Both samples are dissolved in a suitable solvent and spotted onto chromatography paper.

Model answer (5 marks)

Use a suitable solvent as the mobile phase and paper as the stationary phase. The solvent rises by capillary action, carrying the paint components with it. Components that are more soluble in the solvent move further, whereas those that interact strongly with the paper remain closer to the origin. The pure pigment, being a single compound, will give one distinct spot on the chromatogram. The formulation, containing several different pigments and additives, will give multiple spots, each corresponding to a different component. By measuring the Rf values (distance travelled by the spot divided by distance travelled by the solvent front) the individual compounds can be identified and compared between the two samples.

Examiner tips

  • Show the role of the mobile and stationary phases and how differential solubility/affinity causes separation.
  • Mention that the pure pigment gives one spot, the formulation gives several spots.
  • Explain that Rf values are calculated and used for identification.
  • Use correct terminology: mobile phase, stationary phase, Rf, capillary action.

Common mistakes

  • Confusing the solvent with the stationary phase or vice versa.
  • Failing to state that the pure pigment gives a single spot and the formulation gives multiple spots.
  • Not explaining how Rf values are calculated or used for identification.

Mark scheme (5 marks)

  1. The solvent (mobile phase) carries the substances up the chromatography paper (stationary phase)
  2. Substances separate because they differ in how soluble they are in the mobile phase / how strongly they are attracted to the stationary phase
  3. The pure pigment would produce a single spot on the chromatogram
  4. The formulation would produce more than one spot because it contains more than one compound / is a mixture
  5. Rf values (distance moved by substance divided by distance moved by solvent) can be used to identify the individual compounds in each sample

Key terms in this question

paper chromatography · formulation

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