The equilibrium constant is constant when:

• A. Temperature is fixed
• B. Concentrations change
• C. The reaction is not at equilibrium
• D. The rate of the forward reaction equals the rate of the reverse reaction

Answer: (A)

The key idea is that the equilibrium constant is a fixed value for a given temperature. It comes from thermodynamics and relates the concentrations of products and reactants at equilibrium, so K depends on temperature but not on how much of each substance you start with. If you stir in more reactant or remove some product, the system will shift until it reaches a new set of concentrations that still satisfy the same ratio, keeping the same K as long as the temperature is unchanged. If the temperature were to change, K would change as well. The statement about equal forward and reverse rates describes being at equilibrium, but the constancy of K specifically relies on the temperature being fixed.