The hydronium ion notation is convenient in explaining why acids are divided into two categories, strong (complete dissociation in water) and weak (incomplete dissociation).

Two equilibria compete for the proton when a general acid, HA, dissociates into H⁺ and A^-:

One can regard A^- and H₂O as two B-L bases in competition for the proton, with the H₂O having the unfair advantage of being present in great excess.

If the acid anion, A^-, is a stronger B-L base than the water molecule, it will have a greater attraction for protons. It will be able to compete successfully with water moleules for the available protons. Most of the acid then will exist as HA instead of A^-, and we will call HA a weak acid.

In contrast, if H₂O is a stronger B-L base than is A^-, the attraction of water molecules for protons will be greater than that of A^- ions. H₂O will take protons away from HA and form hydronium ions (hydrated H⁺).