A student carries out paper chromatography on a sample of black ink and on a pure dye called Dye X. The black ink produces four separate spots on the paper. Dye X produces only one spot, and its spot appears at the same position as one of the spots from the black ink. Explain what these results tell us about the black ink and about Dye X.

Eduqas GCSE Chemistry — 1.5 Rate of chemical change · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Black ink is a mixture, not a pure substance.
The four separate spots show that the ink contains four different substances (dyes or compounds).
Dye X is a pure substance because it gives only one spot.
The spot of Dye X appears at the same position as one of the ink spots, so Dye X is one of the components present in the black ink.

Examiner tips

  • Use the word "mixture" for black ink and "pure substance" for Dye X. Mention that four spots mean four different components. Explain that the same Rf value shows the same compound in both samples.

Common mistakes

  • Saying the ink is a single compound. Forgetting to state that Dye X is pure. Not linking the same Rf value to the component being present in the ink.

Mark scheme (4 marks)

  1. Black ink is a mixture (not a pure substance)
  2. The four spots show that black ink contains four different substances / compounds / dyes
  3. Dye X is a pure substance because it produces only one spot
  4. Dye X is one of the components / dyes present in the black ink, because its spot is at the same position / has the same Rf value

Key terms in this question

paper chromatography

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