Editing Equilibrium constant (section)

==Water as both reactant and solvent==
For reactions in aqueous solution, such as an acid dissociation reaction 
:AH + H<sub>2</sub>O {{eqm}} A<sup>−</sup> + H<sub>3</sub>O<sup>+</sup> 
the concentration of water may be taken as being  constant and the formation of the [[hydronium ion]] is implicit. 
:AH {{eqm}} A<sup>−</sup> + H<sup>+</sup>
Water concentration is omitted from expressions defining equilibrium constants, except when solutions are very concentrated.
:<math>K=\frac{[A][H]}{[AH]}</math> (''K'' defined as a dissociation constant)
Similar considerations apply to  [[#Hydrolysis constants|metal ion hydrolysis reactions]].