A glass of orange juice, a breath of air and a cup of tap water are all mixtures. Chemists spend a lot of their time pulling mixtures apart, so they can study and use one substance at a time.
To a chemist, a pure substance is made of one kind of particle only. Distilled water is pure: every molecule in it is H₂O. A mixture holds two or more substances together, and each one keeps its own molecules and its own properties. That is what makes a mixture possible to separate: each substance boils, dissolves and sticks in its own way, and this lesson uses each of those in turn.
Most things around you are mixtures. Air is about 78% nitrogen, 21% oxygen and 1% argon, with a little carbon dioxide and water vapour. Tap water carries dissolved minerals. A carton of "pure" orange juice is pure in the shop's sense, with nothing added, and to a chemist it is a mixture of water, sugars, citric acid, vitamin C and hundreds of flavour molecules. Crude oil, the dark liquid a refinery sorts by boiling point, is a mixture of thousands of hydrocarbons.
How chemists tell
A pure substance melts and boils at one sharp temperature. Pure water freezes at 0 °C and boils at 100 °C at sea level. While it boils, the temperature stays at 100 °C until the last of the water has gone, because all the heat goes into pulling the molecules away from each other.
A mixture melts and boils over a range of temperatures. In a solid mixture, the other molecules get in the way of neat packing, so the solid starts to melt at a lower temperature and goes on melting as it warms. Sea water freezes at about −2 °C and boils a little above 100 °C. Candle wax, a mixture of long alkanes, softens slowly as it warms.
This gives chemists a quick test of purity. A pure solid melts sharply, all within about 1 °C, at its own melting point. Even a little of another substance lowers the temperature at which it starts to melt and spreads the melting over several degrees. Chemists compare a sample's melting with the melting point of the pure substance, which is listed in data books. Chromatography, later in this lesson, gives a second test.