Pure Substances

What “pure” means in chemistry

In chemistry, a pure substance is a single element or a single compound, with no other substance mixed into it. A sample containing only iron is pure because iron is one element. A sample containing only water is also pure, even though each water molecule contains both hydrogen and oxygen atoms: water is one compound.

This means that “one type of atom” is not a complete definition of purity. That description works for an element, but a pure compound can contain atoms of several elements chemically bonded together. What matters is that the sample contains only one chemical substance.

A mixture contains two or more substances together. For example, salt water contains water and dissolved sodium chloride, while air contains several gases. The substances in a mixture are not all chemically bonded into one new substance.

Chemical purity and everyday language

The everyday meaning of pure is less precise. It may mean natural, unadulterated or with nothing deliberately added. A label such as “pure milk” can therefore make sense in everyday language, although milk is chemically a mixture containing water, fats, proteins and other substances.

In a chemistry question, use the scientific definition unless the question specifically asks about everyday usage.

Why melting and boiling points reveal purity

The melting point is the temperature at which a solid changes into a liquid. The boiling point is the temperature at which a liquid boils and changes into a gas throughout the liquid.

Under the same conditions, a pure element or compound has a characteristic melting point and boiling point. Because every part of the sample has the same chemical composition, the change of state occurs at a specific temperature. These temperatures can therefore act as characteristic physical properties.

A mixture contains different substances. Its components do not normally all change state at exactly the same temperature, and each can affect the behaviour of the others. Consequently, a mixture generally melts or boils over a range of temperatures, rather than at one sharp temperature.

For a solid, an impurity often makes the melting point lower and broadens the melting range. The safest general conclusion, however, is based on the contrast between a sharp, specific temperature and a temperature range; the precise effect depends on the substances present.

Interpreting melting-point data

Suppose a reference table gives the melting point of a pure substance as $120\ ^\circ\text{C}$.

• If a sample melts sharply at about $120\ ^\circ\text{C}$, its behaviour is consistent with the pure substance.
• If it begins melting at $108\ ^\circ\text{C}$ and finishes at $116\ ^\circ\text{C}$, it melts over a range and is therefore impure or a mixture.
• If it melts sharply at a very different temperature, it may be pure but is unlikely to be the substance named in the reference table.

This shows that melting-point data can provide two kinds of evidence: the width of the melting range helps assess purity, while the value of the melting point can help compare the substance with reference data.

Interpreting boiling-point data

Use boiling-point data in the same way. A pure liquid boils at a specific temperature under fixed conditions, whereas a liquid mixture usually boils over a range because its composition can change as different components enter the gas phase.

For example, if a liquid boils steadily at the reference boiling point, that supports the conclusion that it is pure. If its boiling temperature changes across a range, the evidence indicates a mixture. A single matching value is useful evidence, but conclusions should always be based on the quality and full pattern of the measurements rather than a lone reading.

Definition

A chemically pure substance is one element or one compound, not mixed with anything else. A compound can contain several types of atom and still be pure.

Everyday “pure” may only mean natural or unadulterated; for example, “pure milk” is still a chemical mixture.

Test using change-of-state data

• Pure substance: melts and boils at specific temperatures.
• Mixture or impure substance: usually melts or boils over a temperature range.
• A sharp value matching reference data supports both purity and identity.
• A sharp but different value may indicate a different pure substance.

For solids, impurities often lower the melting point and broaden the melting range. Do not assume that every impurity shifts a boiling point in the same direction.