A food scientist is testing a sports drink to find out whether the orange colouring used in it is a pure substance or a mixture. She carries out paper chromatography on the colouring using water as the solvent. The colouring produces three separate spots on the paper. She then measures the distances travelled and finds that one of the spots has an Rf value of 0.45. Explain what the results of the chromatography tell the scientist about the colouring, and explain how the Rf value could be used to help identify the substance that produced that spot.

WJEC A-Level Chemistry (Wales) — 1.6 Kinetics · Explain · 5 marks · View as Markdown

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

A food scientist carries out paper chromatography on an orange colouring taken from a sports drink. She uses water as the solvent. The colouring produces three separate spots on the chromatography paper. One of the spots travels 2.7 cm while the solvent front travels 6.0 cm.

Model answer (5 marks)

The chromatography shows three distinct spots, so the orange colouring is a mixture, not a pure substance. A pure compound would give only one spot in any solvent, so the presence of three spots proves that more than one compound is present.

The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. Here Rf=2.7 cm/6.0 cm=0.45.

Because different compounds have characteristic Rf values in a given solvent system, the value 0.45 can be compared with published Rf values for known substances run under the same conditions. If a known compound has an Rf of 0.45 in water, that compound is likely the one that produced the spot.

Examiner tips

  • State that three spots mean a mixture; a pure substance gives one spot. Explain the Rf calculation. Mention that Rf values are compared with literature values for identification. Show the numerical calculation to demonstrate understanding.

Common mistakes

  • Claiming that a mixture always gives more than three spots. Forgetting to calculate Rf as distance of spot divided by solvent front distance. Assuming Rf values are universal without specifying the solvent system.

Mark scheme (5 marks)

  1. The colouring is a mixture (not a pure substance)
  2. A pure substance would produce only one spot (in all solvents), so three spots shows it contains more than one compound
  3. The Rf value is calculated as distance moved by the substance divided by distance moved by the solvent
  4. The Rf value of 0.45 can be compared to known / published Rf values of substances (in the same solvent) to identify the compound
  5. Different compounds have different Rf values (in the same solvent), so the Rf value is specific to a particular substance

Key terms in this question

paper chromatography · pure substance · mixture · Rf value

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