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Solids and Liquids are left out when writing the equilibrium expressions because essentially their concentrations stay the same during the reaction. Also, pure substances such as pure liquids and pure solids are left out when writing equilibrium expressions because their concentrations can't change. An example of a pure substance, in particular, a pure liquid, is water.
Liquids are omitted when calculating the equilibrium constant because when there is a solvent, the concentration does not change since there is such a large excess making it insignificant to the calculation. Solids are not used because they do not have molar concentrations.
They are not included because they are not dissolved and hence their concentration is constant or cannot be calculated. Even if you put some more pure solid or some more pure liquid into the solution, you are just increasing the amount of such solid or liquid but their concentration does not change.
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