Except for their names and formulas, so far we have treated all acids as equals. However, acids can be very different in a very important way. Consider HCl(aq). When HCl is dissolved in H₂O, it completely dissociates into H⁺(aq) and Cl⁻(aq) ions; all the HCl molecules become ions:

[latex]\large\text{HCl}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\rightarrow {\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+{\text{Cl}}^{-}\left(aq\right)[/latex] (100% dissociation)

Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid. HC₂H₃O₂ is an example of a weak acid:

[latex]\large \text{H}\text{C}_{2}\text{H}_{3}\text{O}_{2}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\rightarrow {\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+\text{C}_{2}\text{H}_{3}{\text{O}_{2}}^{-}\left(aq\right)[/latex] (~5% dissociation)

Because this reaction does not go 100% to completion, it is more appropriate to write it as an equilibrium:

[latex]\large \text{H}\text{C}_{2}\text{H}_{3}\text{O}_{2}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\rightleftharpoons {\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+\text{C}_{2}\text{H}_{3}{\text{O}_{2}}^{-}\left(aq\right)[/latex]

As it turns out, there are very few strong acids, which are given in Table 1. If an acid is not listed here, it is a weak acid. It may be 1% ionized or 99% ionized, but it is still classified as a weak acid.

The issue is similar with bases: a strong base is a base that is 100% ionized in solution. If it is less than 100% ionized in solution, it is a weak base. There are very few strong bases (see Table 1); any base not listed is a weak base. All strong bases are OH^– compounds. So a base based on some other mechanism, such as NH₃ (which does not contain OH⁻ ions as part of its formula), will be a weak base.