| − | Consider the pair of molecules HX and X<sup>−</sup>. HX can react to form X<sup>−</sup>, and vice versa, through an [[acid-base reaction]]. Thus, HX is the conjugate acid of X<sup>−</sup> (and X<sup>−</sup> is the conjugate base of HX.)
| + | When a base receives a proton, it is called the '''conjugate acid''' of that base. For example, NH<sub>4</sub><sup>+</sup> is the conjugate acid of NH<sub>3</sub>. NH<sub>4</sub><sup>+</sup> and NH<sub>3</sub> are a conjugate acid-base pair. One can be transformed into the other by undergoing a simple [[acid-base reaction]]. |
| − | HX acts as an acid, with a strength inversely proportional to the strength of its conjugate base.
| + | The conjugate acid acts as an acid, with a strength inversely proportional to the strength of its conjugate base. For example, since NH<sub>3</sub> is a weak base, NH<sub>4</sub><sup>+</sup> is a fairly strong acid. |