Editing Chemical equilibrium (section)

==Pure substances==

When pure substances (liquids or solids) are involved in equilibria their activities do not appear in the equilibrium constant<ref name=CEM>{{cite encyclopedia |encyclopedia=Concise Encyclopedia Chemistry |date=1994 |isbn=0-89925-457-8 |url-access=registration |url=https://archive.org/details/conciseencyclope00eagl|title=Biochemistry (2nd Ed.) |last1=Eagleson |first1=Mary }}</ref> because their numerical values are considered one.

Applying the general formula for an equilibrium constant to the specific case of a dilute solution of acetic acid in water one obtains
:CH<sub>3</sub>CO<sub>2</sub>H + H<sub>2</sub>O {{eqm}} CH<sub>3</sub>CO<sub>2</sub><sup>−</sup> + H<sub>3</sub>O<sup>+</sup>

:<math>K_\mathrm{c}=\frac \mathrm{[{CH_3CO_2}^-][{H_3O}^+]} \mathrm{[{CH_3CO_2H}][{H_2O}]}</math>

For all but very concentrated solutions, the water can be considered a "pure" liquid, and therefore it has an activity of one. The equilibrium constant expression is therefore usually written as
:<math>K=\frac \mathrm{[{CH_3CO_2}^-][{H_3O}^+]} \mathrm{[{CH_3CO_2H}]} = K_\mathrm{c}</math>.

A particular case is the [[self-ionization of water]]

:2&nbsp;H<sub>2</sub>O {{eqm}} H<sub>3</sub>O<sup>+</sup> + OH<sup>−</sup>

Because water is the solvent, and has an activity of one, the self-ionization constant of water is defined as

:<math>K_\mathrm{w} = \mathrm{[H^+][OH^-]}</math>

It is perfectly legitimate to write [H<sup>+</sup>] for the [[hydronium ion]] concentration, since the state of [[solvation]] of the proton is constant (in dilute solutions) and so does not affect the equilibrium concentrations. ''K''<sub>w</sub> varies with variation in ionic strength and/or temperature.

The concentrations of H<sup>+</sup> and OH<sup>−</sup> are not independent quantities. Most commonly [OH<sup>−</sup>] is replaced by ''K''<sub>w</sub>[H<sup>+</sup>]<sup>−1</sup> in equilibrium constant expressions which would otherwise include [[hydroxide ion]].

Solids also do not appear in the equilibrium constant expression, if they are considered to be pure and thus their activities taken to be one. An example is the [[Boudouard reaction]]:<ref name=CEM/>

:2&nbsp;CO {{eqm}} CO<sub>2</sub> + C

for which the equation (without solid carbon) is written as:

:<math>K_\mathrm{c}=\frac \mathrm{[CO_2]} \mathrm{[CO]^2}</math>