If we mix a weak acid (HA) with its conjugate base (A^-), both the acid and base components remain present in the solution. This is because they do not undergo any reactions that significanlty alter their concentrations. The acid and conjugate base may react with one another, HA + A^- ? A^- + HA, but when they do so, they simply trade places and the concentrations [HA] and [A^-] do not change. In addition, HA and A^- only rarely react with water. By definition, a weak acid is one that only rarely dissociates in water (that is, only rarely will the acid lose its proton H⁺ to water). Likewise, since the conjugate base A^- is a weak base, it rarely steals a proton H⁺ from water.

So, the weak acid and weak base remain in the solution with high concentrations since they only rarely react with the water. However, they are very likely to react with any added strong base or strong acid.