Pure substances

The meaning of pure

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:

  • a pure substance consists only of one element or one compound
  • a mixture consists of two or more different substances, not chemically joined together
A mixed pile of sweets is separated into four piles of different colours - red, green, yellow and purple.The components of a mixture can usually be separated without chemical reactions

Different types of chemical substance

  • an element contains just one type of atom
  • a compound contains two or more types of atom joined together
  • a mixture contains two or more different substances that are not joined together
  • the different substances in a mixture can be elements or compounds

The table shows some examples:

Table with examples and diagrams of a pure element (oxygen), pure compound (carbon dioxide), mixture of elements (oxygen and helium), mixture of compounds (alcohol and water), mixture of elements and compounds (air)Notice that the different substances in a mixture can be single atoms, molecules of elements or molecules of compounds

Distinguishing between pure substances and mixtures

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.

A cooling curve for salolThe temperature stays the same when a pure substance changes state

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.

Graph of the freezing and melting range of a sunstance, between 40 c and 50 c.The temperature changes slightly as an impure substance changes state
Question

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.