Purity and chromatography · Year 9
In chemistry a pure substance is a single element or compound, not mixed with anything else. Pure substances melt and boil at exact temperatures, so melting points can test purity. A formulation is a useful mixture made to a recipe, such as a paint or a medicine. Paper chromatography separates the substances in a mixture such as an ink, and the Rf value of each spot helps to identify it.
Key facts
- A pure substance has a sharp melting point. Impurities lower the melting point and make it melt over a range of temperatures.
- Formulations are mixtures with measured amounts of each component: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
- Paper chromatography: a spot of the mixture goes on a pencil start line; the solvent (mobile phase) moves up the paper (stationary phase) and carries the substances different distances.
- Rf = distance moved by the substance ÷ distance moved by the solvent. It is always less than 1.
- A pure substance gives one spot in every solvent. Substances with the same Rf in the same solvent are probably the same.
- Draw the start line in pencil (ink would dissolve) and keep the solvent below it.