A scientist uses paper chromatography to investigate a sample of black ink taken from a felt-tip pen. The solvent travels 9.6 cm from the baseline. Three separate spots are produced, travelling 2.4 cm, 6.0 cm, and 8.4 cm from the baseline. The scientist concludes that the black ink is not a pure substance. Explain what the results show about the black ink, and describe how Rf values could be used to help identify the individual substances present.

The black ink is a mixture because it gives three distinct spots rather than one.
Each spot represents a different compound present in the ink.
The compounds separate because they have different affinities for the solvent and the stationary phase – i.e. they have different solubilities and are distributed differently between the mobile and stationary phases.
The Rf value is calculated as the distance travelled by the compound divided by the distance travelled by the solvent front (9.6 cm).
By comparing the Rf values of the spots with reference Rf values for known substances run under the same conditions, each compound can be identified.