A drinks manufacturer produces a new sports drink containing artificial colourings. A trading standards officer wants to check whether the colourings used match the approved list. The officer uses paper chromatography to analyse the drink. One colouring produces a single spot, while another produces three spots. Explain what the results tell the officer about each colouring, and describe how the officer could use Rf values to help identify whether the colourings are on the approved list.

Eduqas A-Level Chemistry — 1.2 Electronic structure · Explain · 5 marks · View as Markdown

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

Paper chromatography separates substances depending on how soluble they are in the mobile phase. The Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.

Model answer (5 marks)

The spot that appears as a single point indicates that the colouring is a pure compound or contains only one detectable substance.
The colouring that gives three distinct spots shows that it is a mixture of three different compounds.
The three compounds separate because they have different solubilities in the mobile phase, so they travel different distances up the paper.
The officer calculates the Rf value for each spot by dividing the distance travelled by the substance by the distance travelled by the solvent.
The calculated Rf values are then compared with the known Rf values of the approved colourings (obtained using the same solvent system) to determine whether the colourings match the approved list.

Examiner tips

  • Use the word ‘pure’ for the single‑spot colouring and ‘mixture’ for the three‑spot one. Show the Rf calculation formula and explain that the comparison must be made with values obtained under identical conditions. Mention that a match of Rf values indicates the same compound.
  • Include the reason for separation – different solubilities – as this shows understanding of chromatography principles.

Common mistakes

  • Confusing the number of spots with the number of compounds; a single spot can still contain impurities. Calculating Rf values incorrectly (e.g. using the wrong distance or not including the solvent front). Failing to recognise that Rf values must be compared using the same solvent system and experimental conditions.

Mark scheme (5 marks)

  1. The colouring that produces a single spot is a pure substance / contains only one compound
  2. The colouring that produces three spots is a mixture / contains three different compounds
  3. The three compounds separate because they have different solubilities in the mobile phase / solvent, so they travel different distances up the paper
  4. The officer calculates the Rf value for each spot by dividing the distance travelled by the substance by the distance travelled by the solvent
  5. The calculated Rf values are compared to known Rf values of approved colourings (in the same solvent) to identify whether they match

Key terms in this question

paper chromatography · Rf value

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