In chemistry, pure substances are investigated. This means substances that are distinguishable from others by specific properties such as melting point, boiling point, density etc.
In nature very few pure substances are found; normally the substances are present in mixtures. Chemists have to separate these mixtures before they can investigate the pure substances.

At the top of the illustration a mixture is shown that contains three pure substances (components). We can classify mixtures according to the physical state and the relative distributions of the components. These physical states of these components can be solid, liquid or gas. We can also distinguish between homogeneous and heterogeneous mixtures according to the nature of their components.
‘Homogeneous’ means that all parts of the mixture have
the same composition. In a homogeneous mixture no separate parts or components can be identified.

In a solution this means that the solute concentration is the same throughout. For a mixture of liquids this means that no liquid-liquid interfaces are visible. For gas mixtures homogeneity means that the composition, density and temperature are the same throughout.

Mixtures are given various names according to their physical states and their composition. The table below gives a simple overview of the mixtures and their common names.

On the left of illustration ST 01, wine is given as an example of a homogeneous liquid mixture (solution). Wine contains, among many other dissolved substances, water and alcohol as major components. The building blocks (molecules) of these two ingredients are shown symbolically on the illustration as spheres and tetrahedrons.

On the right of the illustration orange juice, a heterogeneous mixture (suspension), is depicted. In freshly squeezed fruit juice we find not only liquid components (water + dissolved substances) but also fruit pulp as a solid phase.