Pure substances
A pure substance has a fixed composition and its own set of properties. There are two kinds: elements and compounds. When two or more substances are simply mixed together without joining chemically, we get a mixture instead.
Elements
An element is a substance made of only one type of atom. It cannot be broken down into anything simpler by a chemical reaction. There are just over 100 known elements, each with a chemical symbol – for example oxygen (O), iron (Fe), copper (Cu) and gold (Au). Elements are grouped into metals (such as iron and copper) and non-metals (such as oxygen and sulphur).
Key idea
An atom is the smallest particle of an element that still behaves like that element. An element contains only one kind of atom.
Compounds
A compound forms when two or more elements are chemically combined in a fixed ratio. The atoms are held by chemical bonds, so a compound has properties quite different from the elements that made it. Water (H₂O) is a compound of hydrogen and oxygen; common salt (NaCl) is a compound of sodium and chlorine. A compound can only be separated by a chemical reaction, not by hand.
Mixtures
In a mixture the substances are only physically combined, so each keeps its own properties and the amounts can vary. Air is a mixture of gases, and sea water is a mixture of water and dissolved salts. Because the parts are not chemically bonded, a mixture can be separated by physical methods such as filtering, evaporation or distillation.
Example
Crude palm oil from a Malaysian mill is a mixture. Physical processes separate the oil from water and solids, while a compound like water cannot be split this way – it needs a chemical change such as electrolysis.
| Type | Combined by | Separated by |
|---|---|---|
| Compound | chemical bonds, fixed ratio | chemical reaction |
| Mixture | physical mixing, variable ratio | physical methods |