A chemist uses paper chromatography to separate and identify the dyes in a food colouring mixture. Explain how paper chromatography works and how the chemist can use the results to identify the dyes present.

Eduqas A-Level Chemistry — 3.7 Spectroscopy and chromatography · Explain · 5 marks · View as Markdown

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

Paper chromatography is a technique used to separate mixtures of substances. A spot of the food colouring mixture is placed on the baseline of a piece of chromatography paper and placed in a solvent. A reference sample of known dyes is run alongside the mixture. After the solvent has moved up the paper, the chromatogram is analysed.

Model answer (5 marks)

Paper chromatography separates components by capillary action: the solvent rises up the paper and carries the mixture with it. Different dyes have different solubilities in the solvent and different affinities for the paper, so they travel different distances. Each dye therefore appears as a distinct spot at a particular height above the baseline, giving a separated chromatogram.

The Rf value of a spot is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. The chemist measures the Rf values (or simply the spot positions) of the spots in the unknown mixture and compares them with the Rf values of the reference dyes that were run on the same paper under the same conditions. Where the Rf values (or positions) match, the corresponding dye is identified as present in the mixture.

Examiner tips

  • Use the term "capillary action" and explain the role of solvent and paper. Show the Rf formula and mention that the solvent front is used as the denominator. State that comparison is made with reference dyes run under identical conditions. Include that matching Rf values identify the dyes.

Common mistakes

  • Confusing solvent movement with dye movement; forgetting capillary action. Using the wrong formula for Rf (e.g., distance from baseline instead of solvent front). Not recognising that reference dyes must be run on the same paper and solvent.

Mark scheme (5 marks)

  1. The solvent moves up the paper (by capillary action / is absorbed into the paper)
  2. Different dyes/substances travel different distances up the paper because they have different solubilities in the solvent / different attractions to the paper
  3. Each dye produces a spot at a particular distance from the baseline, giving a separated chromatogram
  4. The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front
  5. The Rf value (or spot position) of each dye in the mixture is compared to the Rf value (or spot position) of the known reference dyes to identify which dyes are present

Key terms in this question

chromatography

Related

More Spectroscopy and chromatography questions

▶ Try answering this question with AI marking (free) →