equilibrium expression

There are two situations for water not to participate in equilibrium expression :

(1) In a heterogeneous equilibrium (it means that the components of the equilibrium are of different phases), water exists as one distinct phase. The effective concentration of water is constant. Therefore, water does not participate in the equilibrium expression.

(2) In an dilute aqueous solution, water is the solvent and is in extremely large excess. When reaction occurs in the dilute solution, the change in concentration of the water solvent is negligible. Therefore, the concentration of water is nearly constant, and water thus does not participate in the equilibrium expression.

The esterification of ethanoic acid and ethanol :
CH3COOH(l) + CH3CH2OH(l) = CH3COOCH2CH3(l) + H2O(l)
At equilibrium, the four components mixed together and form a homogeneous layer. Since water is not in large excess and is miscible in the reaction mixture, water should participate in the equilibrium expression.