Any material that is not a mixture, is called a pure substance. Pure substances include elements and compounds. It is much more difficult to break down pure substances into their parts, and complex chemical methods are needed to do this.
We can use melting and boiling points and chromatography to test for pure substances. Pure substances have a sharply defined (one temperature) melting or boiling point. Impure substances have a temperature range over which they melt or boil. Chromatography is the process of separating substances into their individual components. If a substance is pure then chromatography will only produce one substance at the end of the process. If a substance is impure then several substances will be seen at the end of the process.
Recommended practical activity: Smartie chromatography
You will need:
filter paper (or blotting paper)
some smarties in different colours
an eye dropper.
Place a smartie in the centre of a piece of filter paper. Carefully drop a few drops of water onto the smartie, until the smartie is quite wet and there is a ring of water on the filter paper. After some time you should see a coloured ring on the paper around the smartie. This is because the food colouring that is used to make the smartie colourful dissolves in the water and is carried through the paper away from the smartie.