A student carries out paper chromatography to investigate an unknown food dye. The solvent travels 8.0 cm from the baseline. One spot produced by the unknown dye travels 6.0 cm from the baseline. A known pure dye X has a recorded Rf value of 0.55 in the same solvent. Explain how the student can use the results of the chromatography to determine whether the unknown dye is a pure substance and whether it is the same as dye X.

WJEC A-Level Chemistry (Wales) — 1.7 Equilibria · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A pure substance will give only one spot on the paper. If the unknown dye produced more than one spot the student would know it was a mixture. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent.

Rf (unknown) = 6.0 cm ÷ 8.0 cm = 0.75.

The known dye X has Rf = 0.55. Because 0.75 ≠ 0.55, the unknown dye is not the same as dye X.

Different compounds have different Rf values in the same solvent, so Rf values can be used to identify or distinguish substances.

Examiner tips

  • Show the calculation of the Rf value explicitly.
  • Explain that a single spot indicates purity.
  • Compare the unknown Rf with the known Rf to decide identity.
  • Mention that different compounds give different Rf values.

Common mistakes

  • Calculating Rf incorrectly (e.g. using 8.0 ÷ 6.0).
  • Assuming a single spot automatically means the unknown is dye X without comparing Rf values.
  • Forgetting to state that a pure substance gives only one spot.

Mark scheme (5 marks)

  1. A pure substance produces only one spot in a solvent / a mixture produces more than one spot
  2. Rf value = distance travelled by substance ÷ distance travelled by solvent
  3. Correct calculation of Rf: 6.0 ÷ 8.0 = 0.75
  4. The Rf value of the unknown spot (0.75) does not match the Rf value of dye X (0.55), so the unknown dye is not the same as dye X
  5. Different compounds have different Rf values in the same solvent, so Rf values can be used to identify or distinguish substances

Key terms in this question

Rf value · paper chromatography · pure substance

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