What characterizes a closed system?

A closed system is characterized by the fact that it allows energy exchanges with its surroundings but does not permit matter to enter or leave. This means that while heat or work can cross the boundary of the system, the mass remains constant within that system. This distinction is crucial in thermodynamics, as it helps to analyze energy transformations without the complexity of accounting for changing quantities of matter.

In a closed system, you can observe processes like heat transfer or work being done, but there won't be any mass flow across the system's boundaries. This features also enable straightforward application of the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat supplied to the system minus the work done by the system.

Thus, recognizing that a closed system facilitates energy transfer without the transfer of matter helps in understanding various thermodynamic processes and their implications.