Chromatography

O Level / IGCSE Chemistry · Experimental techniques and chemical analysis. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Paper chromatography separates a mixture of soluble substances, such as the dyes in an ink. A spot of the mixture is put on a pencil line near the bottom of the paper, and the paper stands in a solvent. The solvent level must be below the line. As the solvent rises it carries the substances with it. Each substance travels a different distance, so the mixture spreads out into separate spots.

A pure substance gives one spot. A mixture gives more than one. A spot at the same height as the spot of a known substance is that substance.

Colourless spots are made visible by spraying the paper with a locating agent. Each spot has an Rf value that can be used to identify it.

Rules to remember

Common mistake

Students divide the wrong way round and get an Rf bigger than 1. The solvent always travels furthest, so its distance goes on the bottom of the fraction.

Worked example

On a chromatogram the solvent front is 8.0 cm from the start line. A spot from an unknown dye is 5.2 cm from the start line. In the same solvent, dye J has Rf 0.45 and dye K has Rf 0.65. Which dye is the unknown?

  1. Rf = distance travelled by substance ÷ distance travelled by solvent.
  2. Rf = 5.2 ÷ 8.0 = 0.65.
  3. This matches the Rf value of dye K, not dye J.

Answer: Rf = 0.65, so the unknown dye is K.

Practice questions

Try each one, then open the answer.

1. Why is the start line in chromatography drawn in pencil, not ink?

  1. A
    ink would react with the paper and change the colour of the spots
  2. B
    ink is too dark, so the coloured spots could not be seen on it
  3. C
    ink would dissolve in the solvent and travel up the paper with the spots
  4. D
    pencil is cheaper and easier to rub out if the line is drawn crooked
Show answer

Answer: C. Ink is a mixture of dyes that would move with the solvent and confuse the result; graphite does not dissolve.

2. A dye has an Rf value of 0.60. In an experiment the solvent front moves 7.5 cm. How far does the dye move from the start line?

  1. A
    1.5 cm
  2. B
    4.5 cm
  3. C
    6.9 cm
  4. D
    12.5 cm
Show answer

Answer: B. distance moved by dye = Rf × distance moved by solvent = 0.60 × 7.5 = 4.5 cm.

3. A substance gives a single spot when chromatography is carried out in several different solvents. What does this suggest?

  1. A
    it is colourless
  2. B
    it is a mixture of two substances
  3. C
    it is a pure substance
  4. D
    it is insoluble in all the solvents
Show answer

Answer: C. A mixture would usually separate into more than one spot in at least one of the solvents. One spot every time suggests a pure substance.

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