Pure substances and mixtures

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Why

Almost everything around you is a substance: water, air, metal, milk. To sort substances, first ask: is this a single kind of substance, or are several mixed together?

Rule

Pure substance: A pure substance consists of only one kind of substance, for example pure water or pure oxygen.

Mixture: A mixture consists of several substances, for example air, milk or granite.

A pure substance always has the same properties, for example a fixed boiling point. The substances in a mixture can often be separated again without a chemical reaction, for example by filtering or evaporating.

Pure substances and mixtures compared
Feature	Pure substance	Mixture
Ingredients	one kind of substance	several substances
Examples	pure water, oxygen, table salt	air, milk, granite

Example

Is air a pure substance?

Air consists mainly of nitrogen and oxygen, plus other gases. So several substances are mixed. Air is therefore a mixture, not a pure substance.

Practice

Exercise

Solution

air right answer
Air consists of nitrogen, oxygen and other gases. Several substances are mixed.
milk right answer
Milk contains water, fats and proteins, among other things. It is a mixture.
pure water
Pure (distilled) water consists only of water. It is a pure substance.
pure oxygen
Pure oxygen consists of only one kind of substance. It is a pure substance.

Mixtures consist of several substances. Air and milk are mixtures; pure water and pure oxygen are pure substances.