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For pure solids, you can't find the concentration because it is a solid. For liquids, typically the liquid will be a solvent and therefore, there's a lot of it. If the reaction takes place with some molecules of the solvent, the amount of molecules changed is so insignificant that you can regard it as almost not changed. Therefore, you disregard it.
Pure solids and liquids are not included in the equilibrium expression since their concentrations remain constant throughout the reaction. The density of a pure solid or liquid is the same despite how much pure solid or liquid is present. For the liquids, it is insignificant because since there is alot of solvent, the solute will spread and be part of the liquid. It is like stating, having $1600.10 in your bank account but since the 10 cents is small compared to the total amount available you are well saying $1600 since it won't make much of a difference.
Pure solids and liquids are not included in equilibrium constants because the molar concentration of those substances does not change in reactions. In the case of aqueous solutions, H2O is not included in the constant equation because the overall change in the solvent concentration is insignificant to the reaction as a whole.
Solids don't have a concentration, so they are not placed in the equation. Additionally, the concentration of a liquid doesn't change throughout the reaction, so its like placing a [H2O] on the top and bottom of the Ka equation, and having them cancel out, I believe.
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