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Solids and liquids are pure substances, and their concentrations do not change over the course of the reaction (i.e. do not affect amount of reactant present). Yes, you are correct that solids will not have a concentration because concentration is mol/L, which doesn't make sense for an undissolved solid (a dissolved solid would instead be aqueous (aq) and be included in the equilibrium constant equation). For liquids, they are usually the solvent and in huge excess, so the change in concentration is negligible and approximated to be 0. Therefore, we omit them from K.
Solids don't have concentrations because it doesn't make sense for solids to have a concentration. The concentration of liquids (which are usually solvents) don't change that much during a reaction so it is therefore not included in the equilibrium expression.
Solids and liquids aren't included in the equation because the equation takes into account concentration. It's common sense that solids don't have concentration whereas for liquids, since most are solvents in large excess, it would not really affect the reaction. Therefore, we can just remove solids and liquids from the equation.
Think about it. We are focusing in on an equilibrium reaction between concentrations or gases. If we were to take in account for solids and liquids, we would have to take into account another form of k - muddling up the equation.
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