A student is investigating whether a sample of a food dye is a pure substance or a mixture. She dissolves a small amount of the dye in water and carries out paper chromatography. After the experiment, she observes three separate spots on the paper. Explain what the results of the chromatography tell the student about the food dye, and describe how paper chromatography works to produce this result.

WJEC A-Level Chemistry (Wales) — 1.3 Chemical bonding · Explain · 5 marks · View as Markdown

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

A student carries out paper chromatography on a food dye sample. After the solvent has travelled up the paper, three separate coloured spots are visible.

Model answer (5 marks)

The appearance of three distinct coloured spots shows that the food dye is not a pure substance but a mixture containing three different components.

Paper chromatography separates the components because the paper is the stationary phase and the solvent is the mobile phase. As the solvent rises, it carries the dissolved dye components up the paper. Each component has a different solubility in the solvent; those that are more soluble travel further, while those less soluble travel less far. This difference in solubility causes the components to separate into distinct spots.

Examiner tips

  • State clearly that three spots mean a mixture, not a pure substance. Explain the roles of stationary and mobile phases. Mention that separation is due to differing solubilities and travel distances.
  • common_mistakes
  • :
  • Saying the dye is pure because it has colour. Forgetting to identify the paper as the stationary phase. Claiming separation is due to size rather than solubility.

Mark scheme (5 marks)

  1. Three spots show the food dye is a mixture (not a pure substance)
  2. The food dye contains three different substances / compounds
  3. Paper chromatography involves a stationary phase (the paper) and a mobile phase (the solvent)
  4. The solvent (mobile phase) carries the substances up the paper
  5. The substances separate because they have different solubilities in the solvent / travel different distances because they are more or less soluble

Key terms in this question

paper chromatography · mixture · pure substance

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