Matter and its classification

Matter is anything that has mass and takes up space. Your desk, the air you breathe and you yourself are all matter. It exists in three states:

• Solids have a definite shape and a definite volume.
• Liquids have a definite volume but take the shape of their container.
• Gases have neither: they spread to fill whatever space they are given.

Matter can be sorted in two steps. First: is it a mixture or a pure substance?

A mixture contains two or more pure substances, mixed in any ratio. In a homogeneous mixture the composition is the same throughout: dissolve sugar in water, and every sip is equally sweet. In a heterogeneous mixture it is not: think of sand in water, or a pizza, where you can see the separate toppings. The parts of a mixture keep their own properties, so they can be separated by physical methods such as hand-picking, filtration, crystallisation and distillation.

A pure substance has a fixed composition. Second step: is it an element or a compound?

• An element contains only one kind of atom. Some elements exist as single atoms (sodium, copper); others as molecules of two or more identical atoms joined together (oxygen exists as O₂).
• A compound contains atoms of different elements joined in a fixed ratio. Water is always H₂O. A compound's properties are quite different from those of its elements, and it can be broken down only by chemical methods.

Substances also have two kinds of properties. Physical properties (colour, smell, melting point, boiling point, density) can be observed or measured without changing what the substance is. Chemical properties (how it burns, how acidic it is, how it reacts) show up only in a chemical change.