- The system is the part of the universe that is being studied.
- The boundary is the real or imaginary separation between the system and the rest of the universe.
- The surroundings are the part of the universe outside of the system.
- The universe is everything: the entirety of the system all of the surroundings.
- An open system can exchange energy and matter with the surroundings.
- A closed system can exchange energy, but not matter, with the surroundings.
- An isolated system cannot exchange energy nor matter with the surroundings.
Before we get started, we need to carefully define some terms.
The universe contains everything. If we want to measure something in the universe, we need a way to separate what we are measuring from everything else. We do this by dividing the universe into two parts: the system and the surroundings.
Universe = System + Surroundings
The system is whichever part of the universe we are interested in studying. For example, the system could be chosen as a glass of water, a sealed reaction flask of chemicals, a boat, or the planet Earth. We can use thermodynamics to tell us about the energy and matter within the system, and how different processes would alter it. Once the system is defined, the surroundings are defined as everything else in the universe. The surroundings are usually much bigger than the system.
The boundary between the system and surroundings must be defined to separate the contents (matter and energy) of the system from the surroundings. Like the system, the boundary is chosen (arbitrarily defined) and it is usually conceptual (imaginary) rather than physical. It is crucially important that the boundary is clear so that we can easily say whether energy and/or matter has crossed the boundary.
For example, if the system is defined as a sealed reaction flask of chemicals, we need to establish whether the boundary surrounds the reaction solution only (excluding the flask entirely), or if the boundary is the outside surface of the sealed flask so that the system includes the flask and all of its contents. Either choice is perfectly acceptable, as long as we use it consistently throughout our thermodynamic study.
We can further classify the system based on whether matter and/or energy can cross the boundary between the system and surroundings. There are three types of systems: open systems, closed systems, and isolated systems.
In an open system, both energy and matter may enter or leave the system by crossing the system-surroundings boundary.
In a closed system, energy may be added or removed from the system, but matter cannot cross the boundary.
In an isolated system, neither matter nor energy can cross the boundary.
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