Anion

An ion with a negative charge is called an anion (pronounced an-eye-on). Conversely, an ion with a positive charge is named a cation (pronounced cat-eye-on). Salts contain one or more cations and one or more anions to form a neutral compound. In solution, the cations and anion become solvated and disassociate from each other. An anion can be a single negatively charged atom, such as the halide ions F-, Cl-, Br- and I-, or be comprised of a group of atoms, often containing the electronegative element oxygen. Some anions, such as the cyanide anion, are poisonous. Anions can evolve gasses, act as reducing agents or as oxidizing agents, and these differences are used to test for them in solution. There are 13 common anions which are described below.

Common anions and properties useful for their identification
The presence or absence of each of the thirteen common anions can be detected through a series of five tests, some with several steps, when performed in a particular order. The properties of the anions that are used in such test are indicated in the following table.