When do you add water to the equation and when do you not?
This is more straight forward than you think.
It comes down to how much water have you got. If your whole reaction is happening in water then you have got so much of it that the little bit that you are making is irrelevant and should be left outt of the calculation of Kc. Effectively it comes down to the fact you can't dissolve water in water.
So any aqueous reaction you leave out water, this is most reactions including all Kw and Ka's and most Kc's.
There will be some Kc calculations where you are reacting liquids and you deliberately don't add water. Esterification is the most obvious of these. To get the reaction to work you make sure water is not present (as it is a product and the equilibrium will stay to the left). There are very few others.
Kp is different as this is all about pressure and water, just like everything else, has a pressure as long as it is a gas.
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