Why are solids and liquids not included in Kc expressions?

In equilibrium expressions (like Kc), the concentrations of solids and liquids are not included because their concentrations remain constant throughout the reaction. Solids have a fixed amount and, due to their incompressibility, do not change in concentration as a reaction proceeds. Similarly, the concentration of a pure liquid remains constant because its volume does not change significantly during the reaction.

In equilibrium calculations, Kc is specifically focused on those species that can change their concentration, primarily gases and aqueous solutions, which directly affect the position of equilibrium. Since the concentrations of solids and liquids do not vary in the context of the reaction, they do not contribute to the value of the equilibrium constant.

This constant nature of their concentration allows chemists to simplify the expression by omitting solids and liquids, ensuring that Kc reflects the dynamics of the gases and solutes engaged in the equilibrium state.