A closed system is a system that exchanges energy but no matter with its surroundings. This condition is required for a dynamic equilibrium to be established, since no reactants or products can escape. In an open system, by contrast, matter can be lost, and a true equilibrium may never be reached.

封闭体系是与环境交换能量但不交换物质的体系。建立动态平衡需要这一条件,因为没有反应物或生成物能逸出。与之相对,在开放体系中物质可以散失,因而可能永远达不到真正的平衡。