For example, let us consider the reaction between water and ammonia as represented by the following equilibrium:
In the forward reaction water acts as an acid and donates a proton to ammonia (base). In the reverse reaction ammonium ions (NH4+) act as acid and donate a proton to hydroxyl ions, OH- that act as bases.
The base formed by the loss of proton by an acid is called conjugate base of that acid whereas an acid formed by gain of a proton by a base is called conjugate acid of the base. In the above example, OH- is the conjugate base of H2O and NH4+ is the conjugate acid of NH3. Acid - base pairs such as H2O/OH- and NH4+/NH3 which are formed by loss or gain of a proton are called conjugate acid - base pairs.Water (H2O) acts as both acid and base and such substances are called as amphoteric substances.
