In a single displacement reaction, one reactant replaces an ion of the second reactant. Single displacement reactions occur as A + BC = AC +B. Unless A is more reactive than B, the reaction does not occur.
Chemists also call this type of reaction a single replacement reaction. In the example, A and B are either metals or halogens. When they are metals, A and B represent cations, but if they are halogens, C is the cation in the reaction. Since the oxidation state of C does not change, chemists refer to it as a spectator ion. Such reactions often occur in aqueous solutions and result in precipitation of the metal. If the resulting salt is insoluble in water, it also forms as a precipitate; otherwise, it remains in solution. The single displacement reaction between zinc metal and hydrochloric acid forms hydrogen gas, which escapes the resulting zinc chloride solution. Because the reaction requires changes in charges of the metal or halogen to become an ion and the ion to become an element, all single displacement reactions are also oxidation-reduction reactions. When the reaction occurs with metals, the existing ions and created ions sometimes have different charges, making it necessary to balance the equation. In double replacement reactions, two salts react to form two new salts by exchanging anions and cations.