State function

A state function is a property whose value depends only on a system’s state, not on the path taken to reach it. Its change between two states is independent of the process connecting them.

Thermodynamic state: A macroscopic condition of a system specified by values of properties such as pressure, volume, and temperature. A state function is fixed by the system’s thermodynamic state.
Path independence: A property of an integral whose value depends only on its endpoints, not on the route between them. State-function changes have this defining endpoint-only behavior.
Internal energy: The total microscopic kinetic and potential energy of a system, excluding its bulk motion and external potential energy. Its change is fixed by the initial and final states, even when heat and work vary by path.
Path function: A quantity whose value depends on the process taken between states rather than solely on the states themselves. It is the direct opposite category to a state function.
Thermodynamic equilibrium: A condition in which a system’s macroscopic properties remain constant and no unbalanced driving forces persist. Equilibrium states provide the well-defined endpoints used to compare state-function values.
Thermodynamic cycle: A sequence of processes that returns a thermodynamic system to its initial state. A state function has zero net change over any completed cycle.
Enthalpy: A thermodynamic potential defined as internal energy plus pressure multiplied by volume. It is a state function commonly used to track energy changes at constant pressure.
Reversible process: An ideal process that can be reversed by infinitesimal changes, restoring both system and surroundings without net effects. Even for a reversible process, heat and work remain path quantities rather than state functions.
State variable: A measurable quantity used to specify the condition of a physical system. State variables such as pressure and temperature can determine state functions.
Total differential: A differential expressing the first-order change of a function of several variables. For a state function, its total differential encodes changes in its independent state variables.

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