A factory produces a batch of painkiller tablets. A quality control technician uses paper chromatography to check whether the active ingredient in the tablets is pure. Explain how the technician could use the chromatogram produced to determine whether the active ingredient is pure.

AQA GCSE Chemistry (8462) — 4.8.2 Identification of common gases · Explain · 4 marks · View as Markdown

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

Paper chromatography is used in the pharmaceutical industry to check the purity of compounds in medicines.

Model answer (4 marks)

A pure compound will give a single spot on the chromatogram.
If more than one spot appears the sample is impure or a mixture.
The Rf value of the spot can be measured and compared with the known Rf of the pure active ingredient.
If only one spot is seen and its Rf matches the reference value, the active ingredient is confirmed as pure.

Examiner tips

  • Use the command word ‘Explain’ to describe the steps and reasoning. Mention both the single‑spot criterion and the Rf comparison. Show the link between purity and a single spot.
  • Include the phrase ‘known Rf value’ to match the mark scheme.

Common mistakes

  • Failing to state that only one spot indicates purity. Confusing Rf value with the distance travelled. Ignoring the need to compare with a reference Rf.

Mark scheme (4 marks)

  1. A pure compound produces a single spot (in any solvent)
  2. If more than one spot is produced, the active ingredient is impure / is a mixture
  3. The Rf value of the spot can be compared to the known Rf value of the pure active ingredient
  4. If the Rf value matches the reference / known value (and only one spot is seen), the active ingredient is confirmed as pure

Key terms in this question

chromatogram

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