Purity And Chromatography

A carton in a supermarket promises pure orange juice. A bottle on a bathroom shelf promises pure spring water. A chemist reading either label would tell you, without opening anything, that both claims are false. Not dishonest, just written in a different language. In chemistry the word pure has a single narrow meaning, and almost nothing you eat or drink meets it. The gap between the two meanings is not a curiosity: it is the first mark on this topic, and it is lost every year by candidates who describe a clear colourless liquid as pure because they can see nothing floating in it.

This lesson gives you two ways to hold a substance up to the light and find out what is really in it. The first is a thermometer: a pure solid melts at one sharp temperature and an impure one smears that melting over a range. The second is a strip of paper, a shallow pool of solvent and a pencil line, which will pull a drop of black ink apart into the separate dyes that were hiding inside it. You will learn to run that separation yourself, to read a finished chromatogram, to say which physical method will take a particular mixture apart, and, if you are entered for the Extension Tier, to turn a coloured smudge into a number that names it.