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InkFlaskCambridge IGCSE

Separate · Chemistry

Chromatography paper

What it is for

Paper on which a solvent separates the substances in a mixture.

Mistake to avoid Drawing the start line in ink, or setting the spots below the solvent level: the ink separates too, and the spots dissolve into the solvent.

Drawn in ink: the site's lab kit has no model of it yet, so the one at school may look a little different.

How it is asked

  • Label a chromatogram: the start line in pencil, the spots, the solvent and the solvent front.
  • Interpret it: a pure substance gives one spot; a spot level with a known reference, in the same solvent, identifies that substance.
  • Calculate Rf = distance moved by the substance ÷ distance moved by the solvent (Supplement).

Exam-style question

A student puts a spot of a food colouring and a spot of a blue dye on a pencil start line, then stands the paper in water. The food colouring gives three spots; the blue dye gives one, level with one of them.

  1. What do the results show about the food colouring and the blue dye?
  2. Why is the start line drawn in pencil, above the water level?
  3. The water moves 8.0 cm from the start line and the blue spot 5.2 cm. Calculate the blue dye’s Rf value (Supplement).
Show what a good answer says
  1. The food colouring is a mixture of at least three substances, one of them the blue dye (its spot is level with the blue dye’s). The blue dye gives one spot, so it appears to be pure.
  2. Pencil does not dissolve, so it does not move up with the water as ink would; above the water level the spots move up the paper instead of dissolving into the water.
  3. Rf = 5.2 ÷ 8.0 = 0.65; an Rf value has no unit.

Teaching points for checking your own answer, not official Cambridge marks.

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