Question

which lists all of the substances that appear in equilibrium constant expressions?
pure gas and pure liquid
pure gas and aqueous solution
pure solid and pure liquid
aqueous solution and pure liquid

Explanation:

In equilibrium constant expressions, pure solids and pure liquids are not included because their concentrations (or activities) are considered constant and are incorporated into the equilibrium constant value. Pure gases (with their partial pressures or concentrations) and substances in aqueous solutions (with their molar concentrations) are included as their concentrations can change and affect the equilibrium. So the substances that appear in equilibrium constant expressions are pure gas and aqueous solution.

Answer:

B. pure gas and aqueous solution