The word pure is used in chemistry in a different way from its everyday meaning. For example, shops sell cartons labelled as 'pure' orange juice. The label means that the contents are just orange juice, with no other substances added. However, the juice is not pure in the chemical sense, because it contains different substances mixed together. In chemistry:
The table shows some examples:
Pure substances have a sharp melting point but mixtures melt over a range of temperatures. This difference is most easily seen when the temperature of a liquid is measured as it cools and freezes. The graph shows the cooling curve for a sample of a compound called salol.
The horizontal part of the graph shows that the salol has a sharp melting point, so it is pure. Impure salol (a mixture of salol and other substances) would produce a gradual fall in temperature as it freezes.
A student tests the melting point of a sample of sulfur. It starts melting at 95-101°C but does not melt completely until the temperature is 113°C. According to a data book, the melting point of sulfur is 115°C. Is this sample of sulfur pure or impure?
It is impure, because it melts over a range of temperatures, and the melting point is not the same as the 'standard' reference melting point for sulfur.