Paper chromatography

Spill water on sketch-pen writing and the colour spreads. A black pen may even spread into several colours. That is a clue that black ink is not one colour.

Try paper chromatography:

• Take a strip of chromatography paper (or filter paper) 3 cm wide.
• With a pencil, draw a line 2 cm from the bottom, and put a dot of black ink on it.
• Stand the strip in a thin layer of water. The water must be below the ink dot.

As the water climbs up the paper, it carries the ink with it, and the ink splits into coloured spots. So black ink is a mixture of different dyes.

"Why do the colours separate?" Each dye is pulled differently by the water and by the paper. Some are carried up quickly, others hold on to the paper and move slowly. Different speeds, different heights: the colours spread out. Think of a race where each runner has a different speed: soon they are strung out along the track.

Chromatography can also separate the colours in spinach leaves or flower petals, and in food colours (using a 2% m/v salt solution). Water does not always work; sometimes alcohol or a mixture of solvents is needed.

The name comes from Greek: *chroma (colour) and graphein* (to write), "writing with colour", because it was first used on dyes and inks.