Most elements are rarely found in their pure form. They are found chemically combined with other elements in compounds. Compounds are often found mixed with other compounds. Mixtures may be separated and analysed.
Pure substances have specific melting and boiling points. Mixtures melt and boil over a range of temperatures.
The graphs below show the cooling curves for a pure sample of a compound called salol (C13H10O3) and an impure sample.
We can use these differences between melting and boiling points to distinguish between pure substances and mixtures.