Which of the following statements about an equilibrium system is correct?

Step 1: Analyze the question We need the one true statement about a system in chemical equilibrium.

Step 2: The concept A chemical equilibrium is dynamic. The reactions don't stop. The forward reaction and the backward reaction continue, but at equal rates. As a result, the concentrations of reactants and products stay constant (not necessarily equal).

Step 3: Check each option

• A. Forward and backward reactions occur at the same rate. ✅ True. This is the defining condition of equilibrium.
• B. Concentrations of reactants must equal those of products. ❌ False. They only stay constant. Depending on the equilibrium constant (K), one side may be much larger than the other.
• C. Concentrations can be changed by adding a catalyst. ❌ False. A catalyst speeds up both the forward and backward reactions equally. It helps equilibrium be reached faster but does not change the equilibrium concentrations or the position of equilibrium.
• D. Concentrations are not affected by a change in temperature. ❌ False. Changing temperature shifts the equilibrium position (Le Chatelier's principle) and changes the value of K, so concentrations change.

Step 4: Answer The correct option is A.

Exam tip: Remember "Equal Rates, Constant concentrations" at equilibrium. Rates are equal, concentrations are constant (not equal). A catalyst affects only the speed, never the position. Only changes in temperature, pressure (for gases), or concentration shift the position.