Within the Brønsted–Lowry acid-base theory (protonic), a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton (hydrogen ion). A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton(hydrogen ion) in the forward chemical reaction, hence, the term acid. The base produced, X^-, is called the conjugate base, and it absorbs, or gains, a proton in the backward chemical reaction. In aqueous solution, the chemical reaction involved is of the form