What must NOT be included in a Kc expression?

In a Kc expression, only the concentrations of gases and aqueous solutions are included. This is because the equilibrium constant expression reflects the concentrations of species that can change significantly during the reaction. Solids and liquids have concentrations that do not change appreciably as they do not have a substantial effect on the equilibrium position.

For instance, the concentration of a solid is often considered constant since it does not vary much as the reaction proceeds. Similarly, the same holds for pure liquids. Therefore, when writing a Kc expression, substances in the solid and liquid states are omitted because they do not influence the dynamic balance of the reaction's equilibrium.

This principle is crucial for accurately expressing and calculating equilibrium constants in chemical reactions, allowing scientists to focus on the components that actively participate in the reaction's equilibrium state.