Paper chromatography

Paper is used to separate mixtures of substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Watch this video to understand how paper chromatography can separate a mixtures of soluble substances.

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Big Manny explains how paper chromatography is used to separate mixtures of soluble substances

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Image caption,
Paper chromatography
1. Ink or plant dye is dotted along the pencil line.
Image caption,
2. As the paper is lowered into the solvent, some of the dye spreads up the paper
Image caption,
3. The paper has absorbed the solvent, and the dye has spread further up the paper

Phases

Chromatography relies on two different ‘phases’:

the , which in paper chromatography is very uniform, absorbent paper
the is the that moves through the paper, carrying different substances with it

The different substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper.

Interpreting a chromatogram

Separation by chromatography produces a . A paper chromatogram can be used to distinguish between pure and impure substances:

a pure substance produces one spot on the chromatogram
an impure substance, or mixture, produces two or more spots

Results of a chromatogram of three pure substances and brown ink — Figure caption,
The mixture on the left separated into three substances. The three pure substances made one spot each

A paper chromatogram can also be used to identify substances by comparing them with known substances. Two substances are likely to be the same if:

they produce the same number of spots, and these match in colour
the spots travel the same distance up the paper

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