In a closed system, at chemical equilibrium, what happens to the concentrations of reactants and products over time?

• A. They become constant.
• B. They continually rise.
• C. They continually fall.
• D. They oscillate.

Answer: (A)

At equilibrium in a closed system, the concentrations stop changing with time. This happens because the forward and reverse reaction rates become equal, so the net conversion between reactants and products is zero. Molecules keep reacting in both directions, but the overall amounts of each species remain fixed once equilibrium is reached. Since the system is closed, no matter enters or leaves, so the total amount is conserved and the exact concentrations reflect the temperature, pressure, and the equilibrium constant. If conditions change, the system shifts to a new constant composition, but under fixed conditions the concentrations stay constant.