A student uses paper chromatography to identify the dyes present in a food colouring. The chromatography is carried out using a pencil line as the baseline. After the experiment, the student calculates the Rf value for each spot. Explain how paper chromatography separates the dyes in the food colouring, and state what the Rf value of a substance is.

WJEC A-Level Chemistry (Wales) — 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 can be used to separate and identify substances in a mixture. A pencil baseline is drawn near the bottom of the chromatography paper, and a spot of the mixture is placed on it. The paper is then placed in a solvent, which travels up the paper by capillary action.

Model answer (5 marks)

Paper chromatography separates dyes by capillary action: the solvent rises up the paper carrying the dyes with it. Each dye has a different solubility in the solvent and/or different affinity for the paper, so the more soluble or less attracted dyes travel further up the paper. This differential movement separates the mixture into distinct spots. The Rf value of a substance is the ratio of the distance the substance travels to the distance the solvent front travels, and it is characteristic of that substance under the same experimental conditions.

Examiner tips

  • Use the word ‘capillary action’ to explain solvent movement. Mention differential solubility/affinity for separation. Define Rf as distance travelled by substance ÷ distance travelled by solvent front. State that Rf is characteristic and can identify the dye.

Common mistakes

  • Confusing Rf with the solvent front distance. Saying the solvent carries the dyes but not that separation is due to differing solubilities. Using vague terms like ‘moves’ without explaining capillary action or affinity.

Mark scheme (5 marks)

  1. The solvent travels up the paper, carrying the dyes with it.
  2. Different dyes have different solubilities in the solvent (or different attractions to the paper/solvent).
  3. More soluble dyes travel further up the paper, separating the mixture into distinct spots.
  4. The Rf value is the distance travelled by the substance divided by the distance travelled by the solvent front.
  5. The Rf value is unique to a substance (under the same conditions) and can be used to identify the dye.

Key terms in this question

chromatography · Rf value · baseline

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