Buffer solutions resist changes in pH upon the addition of a small amount of acid or base. This is because they contain both a weak acid and its conjugate base (or a weak base and its conjugate acid). When a small amount of strong acid ($H^+$) or strong base ($OH^-$) is added, it reacts with the conjugate base or weak acid present in the buffer to form unionised (or weakly ionised) acid or base, thereby neutralizing the added ions and maintaining the pH relatively constant .