定义Definition
动态平衡是可逆反应在封闭体系中正反应速率与逆反应速率相等时的状态。此时各物质的浓度保持不变，但正、逆反应仍在不断进行，只是互相抵消，宏观上看不出变化。
Dynamic equilibrium is the state of a reversible reaction in a closed system when the forward and reverse rates are equal. The concentrations then stay constant, but both reactions continue, cancelling each other so that no overall change is seen.
条件与特征Conditions and features
只能在封闭体系中建立（物质不能进出）；从正向或逆向开始都能达到同一平衡；温度不变时平衡组成不变；平衡时各物质浓度不一定相等。
It can be reached only in a closed system, where nothing enters or leaves; the same equilibrium is reached from either direction; at constant temperature the composition does not change; and at equilibrium the concentrations need not be equal.
“动态”的证据Evidence that it is dynamic
向已达平衡的体系中加入含同位素标记的物质（如含 ¹⁸O 的水或放射性碘），一段时间后标记原子出现在所有相关物质中，说明正、逆反应一直在进行。
Adding an isotopically labelled substance to a system at equilibrium, such as water containing ¹⁸O or radioactive iodine, the label soon spreads into all the species involved, showing that the forward and reverse reactions are still going on.