A student uses paper chromatography to investigate the dyes present in two food colourings, labelled P and Q. Food colouring P produces three spots on the paper. Food colouring Q produces one spot. The student claims that food colouring Q is a pure substance. Explain how paper chromatography separates the dyes in a mixture, and evaluate whether the student's claim about food colouring Q is correct.

In paper chromatography the solvent is the mobile phase and the paper is the stationary phase. As the solvent rises, it carries the dyes up the paper. Dyes that are more soluble in the solvent move further, so dyes with different solubilities separate into distinct spots.

Food colouring P gives three spots, so it contains at least three different dyes – it is a mixture. Food colouring Q gives only one spot, so in this single test it behaves like a pure compound.

However, a single spot in one solvent does not prove that Q is pure. A pure substance will give one spot in every solvent tested, but a mixture of dyes that are very similar in solubility could also give a single spot in a particular solvent. Therefore the claim that Q is pure is not conclusively supported by the data and should be verified with additional solvents.