Systems

In thermodynamics, the universe is divided into two parts whenever we analyze a reaction or process: the system, the specific part of the universe being focused on — a chemical reaction, a sample of gas, or any defined quantity of matter — and the surroundings, everything else outside the system, such as the container, the room, or the larger environment. For example, in a chemical reaction, the system is the reactants and products involved; the surroundings are the reaction container, the air in the room, and anything else external to the reaction itself. The boundary separates the system from its surroundings, and its nature determines how energy and matter are exchanged between the two.

Key Takeaways

• A system is the part of the universe being analyzed; the surroundings are everything else; the boundary separates them and governs what can be exchanged.

• Isolated systems exchange neither energy nor matter; closed systems exchange energy only; open systems exchange both.

• State functions (temperature, pressure, volume, energy, enthalpy) depend only on a system's current state.

• Path functions (heat, work) depend on the process taken between states — they are not properties of the system itself.

Types of Systems

Based on what can cross the boundary, systems fall into three categories:

• Isolated system — no exchange of energy (heat or work) or matter with the surroundings. Example: a bomb calorimeter, designed to prevent any exchange with the outside environment.

• Closed system — energy can be exchanged with the surroundings, but matter cannot. Example: a steam radiator, where heat transfers to the surroundings but the water stays inside the system.

• Open system — both energy and matter can be exchanged with the surroundings. Example: a pot of boiling water, where heat transfers out and water escapes as steam.

State Functions vs. Path Functions

A state function is a property of a system that depends only on its current state — not on how that state was reached. State functions are path-independent, meaning they depend only on the system's initial and final states. Examples include temperature, pressure, volume, energy, and enthalpy.

A path function, by contrast, depends on the specific process or path taken to move between states. Heat and work are path functions: they describe energy transfer along the path, not a property of the system at any given moment.

MCAT Callout — Why Heat and Work Aren't State Functions: Heat and work are involved in the process of change, but they don't define a system's equilibrium states — they aren't part of the system's condition at the start or end of a process, only the transfer that happens in between. That's why the same change in a system's state (say, the same temperature increase) can be achieved through different combinations of heat and work, even though the state functions describing the system before and after are identical.

In short: state functions define the equilibrium properties of a system, while path functions like heat and work describe the processes that lead to those equilibrium states.

Common MCAT Mistakes

• Confusing closed and isolated systems. A closed system can still exchange energy (heat or work) with its surroundings — only an isolated system blocks both energy and matter.

• Assuming boiling water is a closed system. Because steam escapes into the surroundings, matter leaves the system, making it open, not closed.

• Treating heat or work as a property of a system's state. Heat and work are path functions describing a transfer process, not equilibrium properties like temperature, pressure, or volume.

• Assuming the same state change means identical heat and work. Different combinations of heat and work can produce the same net change in state functions, since heat and work are path-dependent even though the resulting state change is path-independent.

MCAT-Style Concept Check

Question: A steam radiator releases heat into a room, but the sealed radiator itself never lets water or steam escape. Based on this, how should the radiator be classified?

• A) Isolated system, because no matter crosses the boundary

• B) Closed system, because energy crosses the boundary but matter does not

• C) Open system, because heat is a form of matter

• D) Isolated system, because temperature is a state function

Answer: B

Explanation: The radiator exchanges energy (heat) with the room but keeps its water sealed inside, so no matter crosses the boundary. That combination — energy exchanged, matter not — defines a closed system. Option A is wrong because isolated systems block energy exchange too, which the radiator clearly doesn't. Option C misclassifies heat as matter. Option D correctly notes temperature is a state function but that fact doesn't bear on how the system is classified.

FAQ

What's the difference between a system and its surroundings?

The system is the specific part of the universe under study — such as the reactants and products in a reaction. The surroundings are everything else outside it, like the container, room, or environment. The boundary separates the two and governs what can be exchanged between them.

What's the difference between isolated, closed, and open systems?

An isolated system exchanges neither energy nor matter with its surroundings (e.g., a bomb calorimeter). A closed system exchanges energy but not matter (e.g., a steam radiator). An open system exchanges both energy and matter (e.g., a pot of boiling water).

What makes something a state function instead of a path function?

A state function depends only on a system's current state, not on how that state was reached — examples include temperature, pressure, volume, energy, and enthalpy. A path function depends on the specific process or path taken between states — heat and work are the two classic examples.

Why aren't heat and work considered state functions?

Because they describe energy transfer that occurs during a process of change, not a property that defines the system's equilibrium state at the start or end. The same net change in a system's state can be reached through different combinations of heat and work, even though the state functions before and after remain identical.