Editing Hydrolysis (section)

===Metal aqua ions===
{{Main|Metal ions in aqueous solution}}
Metal ions are [[Lewis acid]]s, and in [[aqueous solution]] they form [[metal aquo complex]]es of the general formula {{chem2|M(H2O)_{''n''}^{''m''+} }}.<ref>{{cite book |last1=Burgess |first1=John |title=Metal Ions in Solution |date=1978 |publisher=Ellis Horwood |location=Chichester |isbn=978-0853120278}}</ref><ref>{{cite book |last= Richens |first= D. T. |title= The Chemistry of Aqua Ions: Synthesis, Structure, and Reactivity: A Tour through the Periodic Table of the Elements |publisher= Wiley |year= 1997 |isbn= 0-471-97058-1}}</ref> The aqua ions undergo hydrolysis, to a greater or lesser extent. The first hydrolysis step is given generically as

:{{chem2 | M(H2O)_{n}^{m+} + H2O <-> M(H2O)_{n-1}(OH)^{(m-1)+} + H3O+ }}

Thus the aqua [[cation]]s 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 {{chem2|O\sH}} bond of an attached water molecule, making the liberation of a proton relatively easy.

The [[acid dissociation constant|dissociation constant]], pK<sub>a</sub>, for this reaction is more or less linearly related to the charge-to-size ratio of the metal ion.<ref name="bm">{{cite book |last1=Baes |first1=Charles F. |last2=Mesmer |first2=Robert E. |title=The Hydrolysis of Cations |date=1976 |publisher=Wiley |location=New York |isbn=9780471039853}}</ref> Ions with low charges, such as {{chem2|Na+}} are very weak acids with almost imperceptible hydrolysis. Large divalent ions such as {{chem2|Ca(2+)}}, {{chem2|Zn(2+)}}, {{chem2|Sn(2+)}} and {{chem2|Pb(2+)}} have a pK<sub>a</sub> of 6 or more and would not normally be classed as acids, but small divalent ions such as {{chem2|Be(2+)}} undergo extensive hydrolysis. Trivalent ions like {{chem2|Al(3+)}} and {{chem2|Fe(3+)}} are weak acids whose pK<sub>a</sub> is comparable to that of [[acetic acid]]. Solutions of salts such as {{chem2|BeCl2}} or {{chem2|Al(NO3)3}} in water are noticeably [[acidic]]; the hydrolysis can be [[Le chatelier's principle|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]].<ref name="bm"/> Some "exotic" species such as {{chem2|Sn3(OH)4(2+)}}<ref>{{Greenwood&Earnshaw|page=384}}</ref> are well characterized. Hydrolysis tends to proceed as [[pH]] rises leading, in many cases, to the precipitation of a hydroxide such as {{chem2|Al(OH)3}} or {{chem2|AlO(OH)}}. These substances, major constituents of [[bauxite]], are known as [[laterite]]s 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.