The reactions of either iron or zinc combined with hydrochloric acid are two prime examples of single replacement reactions, also known as single displacement or substitution reactions. In these reactions, one element combines with a compound, where the single element replaces one of the other elements in the compound.
Single replacement reactions always begin and end with a compound and a single element. In the example reactions of zinc or iron combining with hydrochloric acid, the metal replaces the hydrogen in the hydrochloric acid, producing either zinc or iron chloride and hydrogen.
Single replacement reactions can only happen when a more reactive element is taking the place of a less reactive element. In the examples, zinc and iron are more reactive than the hydrogen and thus take its place in the compound. Most single replacement reactions occur when a metallic element is placed in an aqueous solution. In order to determine if this type of reaction can occur, it is necessary to look at a table of the reactivity of the specific elements to decide whether or not the single element is more reactive than the elements in the compound. As the less reactive element cannot retake its place from the more reactive element, this type of reaction is always irreversible.