Metal ions are Lewis acids, and in aqueous solution they form metal aquo complexes of the general formula . The aqua ions undergo hydrolysis, to a greater or lesser extent. The first hydrolysis step is given generically as
Thus the aqua cations behave as acids in terms of Brønsted–Lowry acid–base theory. This effect is easily explained by considering the inductive effect of the positively charged metal ion, which weakens the  bond of an attached water molecule, making the liberation of a proton relatively easy.
The dissociation constant, pK, for this reaction is more or less linearly related to the charge-to-size ratio of the metal ion. Ions with low charges, such as  are very weak acids with almost imperceptible hydrolysis. Large divalent ions such as , ,  and  have a pK of 6 or more and would not normally be classed as acids, but small divalent ions such as  undergo extensive hydrolysis. Trivalent ions like  and  are weak acids whose pK is comparable to that of acetic acid. Solutions of salts such as  or  in water are noticeably acidic; the hydrolysis can be suppressed by adding an acid such as nitric acid, making the solution more acidic.
Hydrolysis may proceed beyond the first step, often with the formation of polynuclear species via the process of olation. Some "exotic" species such as  are well characterized. Hydrolysis tends to proceed as pH rises leading, in many cases, to the precipitation of a hydroxide such as  or . These substances, major constituents of bauxite, are known as laterites and are formed by leaching from rocks of most of the ions other than aluminium and iron and subsequent hydrolysis of the remaining aluminium and iron.