A student uses paper chromatography to separate the coloured pigments in a sample of black ink. After running the chromatogram, five separate spots are visible. The solvent front has travelled 9.0 cm. One spot has travelled 6.3 cm and another has travelled 1.8 cm. Explain how the student could use the chromatogram to identify whether any of the pigments in the black ink are pure substances, and describe how Rf values are calculated and used in this identification.

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 used to separate mixtures. Each component in a mixture can be identified by calculating its Rf value and comparing it to known reference values.

Model answer (5 marks)

The student can calculate the Rf value for each spot by dividing the distance the spot travelled by the distance the solvent front travelled. For example, for the spot that travelled 6.3 cm: Rf = 6.3 cm ÷ 9.0 cm = 0.70. For the spot that travelled 1.8 cm: Rf = 1.8 cm ÷ 9.0 cm = 0.20. The calculated Rf values are then compared with reference Rf values for known pure pigments. If a calculated Rf matches a reference value, that spot is identified as that pure pigment. A pure substance will give only one spot on the chromatogram; it will not split into multiple spots. Because five distinct spots are observed, the ink contains at least five different substances and is therefore not a pure pigment.

Examiner tips

  • Show the Rf formula and perform at least one calculation; include units (cm/cm).
  • Explain that a single spot indicates a pure component; multiple spots mean a mixture. Keep the explanation concise and use the term "reference Rf values".

Common mistakes

  • Using the wrong distance (e.g., subtracting the spot distance from the solvent front).
  • Failing to state that a pure substance gives only one spot.

Mark scheme (5 marks)

  1. Rf value is calculated by dividing the distance travelled by the substance (spot) by the distance travelled by the solvent front
  2. Correct calculation of at least one Rf value — 6.3 ÷ 9.0 = 0.70 or 1.8 ÷ 9.0 = 0.20
  3. The calculated Rf value is compared to Rf values of known, pure substances (reference/standard values)
  4. A pure substance produces only one spot / gives a single spot on the chromatogram (does not separate further)
  5. Since five spots are visible, the black ink is a mixture of at least five different substances / is not a pure substance

Key terms in this question

Rf value · solvent front · chromatography · pure substance

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