A scientist is testing two dye samples, P and Q, using paper chromatography with the same solvent. Dye P produces a single spot. Dye Q produces three spots. The spot from dye P travels 4.8 cm and the solvent front travels 6.0 cm. One of the spots from dye Q also travels 4.8 cm. Explain what the results tell the scientist about the nature of dye P and dye Q, and explain how the Rf value could be used to help identify the substance in dye P.

Eduqas A-Level Chemistry — 1.7 Equilibria · Explain · 5 marks · View as Markdown

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

A scientist uses paper chromatography to analyse two dye samples, P and Q, using the same solvent. Dye P produces a single spot that travels 4.8 cm up the paper. Dye Q produces three separate spots. The solvent front travels 6.0 cm in both cases. One of the spots in dye Q also travels 4.8 cm.

Model answer (5 marks)

Dye P gives a single spot, so it is a pure substance (single compound). Dye Q gives three spots, so it is a mixture of at least three different compounds. The Rf value of dye P is 4.8 cm ÷ 6.0 cm = 0.80. This Rf can be compared with reference Rf values for known dyes in the same solvent system to identify the compound in dye P. Because the spot in dye Q that also travels 4.8 cm has the same Rf (0.80), the compound in dye P is likely the same as that spot in dye Q.

Examiner tips

  • State that a single spot = pure; multiple spots = mixture. Show the Rf calculation. Mention comparison with reference Rf values. Note that identical Rf suggests same compound.

Common mistakes

  • Calculating Rf incorrectly (e.g., 6.0/4.8).
  • Failing to explain that identical Rf indicates same compound.

Mark scheme (5 marks)

  1. Dye P is a pure substance (single compound/element)
  2. Dye Q is a mixture because it produces more than one spot
  3. The Rf value for dye P is calculated as distance travelled by substance divided by distance travelled by solvent (4.8 ÷ 6.0 = 0.8)
  4. The Rf value can be compared to known/reference Rf values to identify the substance in dye P
  5. The substance in dye P is likely to be the same compound as the substance producing the spot at 4.8 cm in dye Q, because they have the same Rf value (in the same solvent)

Key terms in this question

Rf value · paper chromatography

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