Using melting point and boiling point to test purity

A pure substance melts and boils at specific temperatures under a given pressure. Its melting point and boiling point are therefore characteristic physical properties that can be compared with known values.

To test a sample, measure its melting point or boiling point and compare the result with the known value for the substance under the same conditions. If the measured value agrees within experimental uncertainty, the result is consistent with the sample being pure. If it differs, the sample may contain impurities or may be a mixture.

An impure sample or mixture does not necessarily change state at the same temperature as the corresponding pure substance. It may also melt or boil over a range of temperatures rather than at a single temperature. A melting or boiling range can therefore be evidence that a sample is impure.

For example, pure water melts at $$0,^{\circ}\mathrm{C}$$ and boils at $$100,^{\circ}\mathrm{C}$$ at standard atmospheric pressure. A sample expected to be pure water that boils at a noticeably different temperature under those conditions may contain impurities.

A matching value alone does not prove purity: measurement uncertainty and the behaviour of some mixtures can make an impure sample appear to have the expected value. Pressure must also be considered, especially when comparing boiling points.

Pure substances in chemistry

In chemistry, a pure substance contains only one kind of particle. It consists of a single element or a single compound, with no other substance mixed with it.

A pure element contains only one type of atom. A pure compound contains only one type of substance formed from different elements chemically bonded together. A compound can therefore be pure even though it contains more than one type of atom.

Chemical purity and everyday language

In everyday language, pure may mean that nothing has been added to a material or that it is natural. For example, milk may be called pure even though it contains several substances mixed together. Chemically, it is a mixture, not a pure substance.

Chemical purity is about whether a material consists of a single substance, not whether it is natural or untreated.