Water in Chemical Equilibria:
In general diluted aqueous solutions are studied: water (H2O)
will be in excess and its activity may be considered to be constant,
and therefore set equal to one
(log activity = 0).
However, water (H2O) might be needed as a component if you wish
to model ionic medium effects, especially on systems where minerals precipitate
involving H2O, such as gypsum
(CaSO4·2H2O).
Therefore it is recommended that
H2O is included in the reactions.