The classification of systems, as open and closed, is based on the ability of a system to interact with the environment. A closed system is one which dies not interact with its environment. It neither receives nor donates anything to or from its environment. In other words, it is separated from its environment. Closed systems that do not even exchange energy with the surroundings are called isolated systems and are a special class of closed systems. There are very few truly closed systems in nature. One can think of a sealed jar of rotting food material as closed system, since it will not exchange gases (or anything) with its surroundings. In comparison, an open system is open to the environment and is free to exchange mass and energy with its surroundings. A river exchanges gases with the atmosphere and maintains a dissolved oxygen level. A cup of tea exchanges heat with the surroundings and gets cold with time.
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