Which of the following is included in the equilibrium constant expression?

The equilibrium constant expression is derived from the law of mass action, which states that at a given temperature, a chemical reaction reaches a state where the rates of the forward and reverse reactions are equal. In this expression, only the concentrations (or partial pressures) of gaseous reactants and products and the concentrations of substances in aqueous solution are included. This is because these states can change in response to variations in conditions, which affects the position of the equilibrium.

Pure solids and pure liquids are not included in the equilibrium constant expression because their concentrations remain constant throughout the reaction. For example, the amount of a solid or liquid present does not change in a way that would affect the ratio of products to reactants at equilibrium, as their densities and volumes are effectively constant and do not contribute to the dynamic nature of the equilibrium. As a result, the term for pure solids or liquids is considered to be equal to 1 and does not appear in the expression.

The correct focus on gases and aqueous solutions reflects their ability to vary in concentration or pressure and thus directly influence the reaction's equilibrium position. Therefore, the presence of only gases and aqueous solutions in the equilibrium constant expression embodies the factors that alter the reaction rates essential for determining equilibrium.