Purity and formulations

In everyday language, pure means nothing has been added — "pure orange juice" is just juice, in its natural state.

In chemistry, a pure substance is a single element or a single compound, not mixed with anything else. Orange juice is a mixture of water, sugars, acids and more, so to a chemist it isn't pure at all. Pure water contains only water molecules.

A pure substance melts and boils at a specific temperature — it has a sharp melting point. Pure water melts at exactly 0 °C and boils at 100 °C (at normal pressure).

An impure substance (a mixture):

• melts over a range of temperatures
• usually melts at a lower temperature than the pure substance
• usually boils at a higher temperature

So measuring the melting point is a quick way to check purity. If a sample melts sharply at the expected temperature, it is likely to be pure.

A formulation is a mixture designed as a useful product. Each ingredient is added in a carefully measured quantity and has a particular job, so that the product has exactly the properties needed.

Examples include fuels, cleaning products, paints, medicines, alloys, fertilisers and foods.

A medicine, for example, contains the active drug plus other ingredients to give it a pleasant taste, make it into a tablet, or help it dissolve at the right rate.