Thermodynamic state

A thermodynamic state is the condition of a system specified by its macroscopic properties, such as temperature, pressure, volume, and composition. For an equilibrium system, these properties determine its thermodynamic behavior.

State variable: A measurable property that specifies a system’s condition or helps describe its evolution. Temperature and pressure are state variables used to describe thermodynamic states.
Thermodynamic process: A change in a thermodynamic system from one state to another. A process is the route connecting an initial state to a final state.
Nonequilibrium thermodynamics: The study of systems with gradients, flows, or other departures from thermodynamic equilibrium. A nonequilibrium system may lack a single set of uniform state variables.
Phase diagram: A chart showing which phases are stable under different conditions, such as temperature and pressure. It maps regions of state space to the phases a substance can occupy.
Thermodynamic equilibrium: A condition in which a system’s macroscopic properties do not change spontaneously over time. Equilibrium states are the stable conditions most often described by thermodynamic state variables.
Thermodynamic path: The sequence of intermediate states followed during a thermodynamic process. Different paths can connect the same pair of states.
Local thermodynamic equilibrium: An approximation in which small regions of a nonequilibrium system have well-defined equilibrium properties. It extends state-variable descriptions to systems that are not globally at equilibrium.
Phase transition: A change between distinct physical phases, driven by varying conditions such as temperature or pressure. It marks where a change in state variables alters the stable phase.
Equation of state: A relation connecting the state variables of a material or system. It constrains which combinations of pressure, volume, and temperature are possible.
Thermodynamic cycle: A sequence of thermodynamic processes that returns a system to its initial state. The cycle closes because the final state equals the initial state.

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