A food scientist is investigating three sports drinks — X, Y and Z. She wants to find out whether each drink is a pure substance or a formulation, and to identify some of the colourings present. She carries out paper chromatography on all three drinks using the same solvent. Drink X produces a single spot. Drink Y produces three spots. Drink Z produces the same single spot as drink X, at the same position on the paper. Explain what the chromatography results tell the food scientist about drinks X, Y and Z, and explain how the Rf value could be used to help identify the colouring in drink X.

WJEC A-Level Chemistry (Wales) — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Drink X is a pure substance because it gives only one spot on the paper.
Drink Y is a formulation (mixture) because it gives three distinct spots, showing more than one compound.
Drink Z contains the same colouring as drink X because its single spot travels to the same position as the spot from X, giving the same Rf value.
The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front.
The Rf value of the spot from drink X can be compared with reference Rf values for known colourings run in the same solvent system to identify the colouring present.

Examiner tips

  • Use the command word ‘Explain’ – give a short reason for each conclusion. State the definition of Rf and show the formula. Mention that identical Rf values in the same system indicate the same compound. Show how a reference Rf list can be used to identify the colouring.

Common mistakes

  • Confusing a single spot with a mixture – forgetting that a single spot indicates a pure compound. Calculating Rf incorrectly (e.g. using solvent distance instead of spot distance). Assuming the same Rf always means the same compound without noting that the solvent system must be identical.

Mark scheme (5 marks)

  1. Drink X is a pure substance / contains only one compound (as it produces a single spot)
  2. Drink Y is a mixture / formulation (as it produces more than one spot / three spots, showing it contains more than one compound)
  3. Drink Z contains the same colouring / compound as drink X (because the spot travels to the same position / has the same Rf value in the same solvent)
  4. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
  5. The Rf value of the spot from drink X can be compared with known/reference Rf values (for known colourings in the same solvent) to identify the colouring

Key terms in this question

pure substance · formulation · Rf value · paper chromatography

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