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An open system exchanges both matter and energy with its environment, like a moving car whose engine combusts fuel and releases exhaust gases. A closed system exchanges only energy, not matter, with its surroundings, such as a car coasting to a stop due to friction. Both differ from isolated systems, which exchange neither matter nor energy.

Thermal equilibrium occurs when a system and its surroundings reach the same temperature through heat exchange. For example, hot tea in a thermos gradually cools until it matches the surrounding temperature. At this point, the Zeroth Law of Thermodynamics requires that temperature be uniform throughout the system, and no further net heat transfer occurs.

An isolated system exchanges neither matter nor energy with its environment. A thermos containing hot tea initially acts as an isolated system because the lid prevents steam escape and insulation minimizes heat loss. However, no system remains truly isolated indefinitely; the tea eventually cools, demonstrating that energy exchange eventually occurs.

The boundary separates the system from its environment and determines what can be exchanged. Through this boundary, the system and environment can exchange heat and do work on each other. The immediate surroundings directly influence the system's behavior and properties more strongly than distant environmental factors.

Open systems allow matter to cross their boundaries, such as steam escaping from a boiling kettle with an open lid. Closed systems prevent matter exchange but permit energy transfer, like a sealed kettle where steam cannot escape but heat is exchanged. This distinction determines how the system's composition changes over time.

The Zeroth Law establishes that thermal equilibrium requires uniform temperature throughout a closed system. This law is essential for studying heat transfer processes between a system and its surroundings. It provides the foundation for understanding how systems reach equilibrium and defines the conditions necessary for thermodynamic analysis.

No real system can remain isolated indefinitely. Even a well-insulated thermos eventually exchanges energy with its surroundings as the contained tea cools to match the ambient temperature. This demonstrates that perfect isolation is theoretically impossible, and all systems eventually interact with their environment through heat transfer or other mechanisms.