[mahela007]

The common answer (and the one in my text book) to this is that the "concentration" of the water in the solution varies so little that it can be considered constant and is therefore incorporated into the equilibrium constant.
However, in writing K_c expressions for other reactions, the reason for not including a liquid or solid in the equilibrium expression is that the concentration of a solid or a pure liquid cannot change.
This seems to imply that in the case of the auto-ionization of water, the concentration of water changes but it ignored and that in other reactions, the concentration of pure water does not change at all.
Have I understood this correctly or have I gone wrong somewhere?

[ContinuousProcess]

Ya you pretty much got it right.

But the concentration of water can't change, it's the number of particles of dissociated water that changes. Water is still the solvent, so its concentration cannot change. (Technically concentration of the solution does change, but so little solvent converts into 'solute' that the overall volume of solvent is the same in all practicality).

Concentration describes the amount of solute in reference to an amount of solvent; which is why it is irrelevant in pure liq+sol phase.

[Borek]

This was discussed many times, please search the forum.

http://www.chemicalforums.com/index.php?topic=43091.0