Single replacement is a type of chemical reaction where one element replaces another element in a compound. This process typically occurs when a more reactive element displaces a less reactive one. For example, in the reaction of zinc with hydrochloric acid, zinc replaces hydrogen to form zinc chloride and hydrogen gas. In a single replacement reaction, the general formula can be represented as A + BC → AC + B. Here, A is the free element, and BC is the compound. This reaction is common in various applications, including metal extraction and the production of gases.