In a reversible reaction at chemical equilibrium, which condition holds true?

• A. The forward and reverse rates are equal.
• B. The concentrations of reactants and products are equal.
• C. The reaction has stopped completely.
• D. Temperature must be constant.

Answer: (A)

At chemical equilibrium for a reversible reaction, the forward and reverse rates become equal. This means the same amount of material is moving from reactants to products as from products back to reactants per unit time, so the overall concentrations stop changing even though both directions continue to occur. The actual concentrations of reactants and products don’t have to be equal; their ratio is set by the temperature through the equilibrium constant. Temperature must stay constant to keep this balance, because changing the temperature would alter the rates and the equilibrium position. The key point is the rate equality, which defines equilibrium, while the other statements describe consequences or conditions that are not the defining feature.