**Adiabatic**—condition of zero heat interaction at system boundaries.

**Association**—description of complex formation where all molecules in the complex are of the same type.

**Azeotrope**—mixture which does not change composition upon vapor-liquid phase change.

**Barotropy**—the state of a fluid in which surfaces of constant density (or temperature) are coincident with surfaces of constant pressure.

**Binodal**—condition of binary phase equilibrium.

**Dead state**—a description of the state of the system when it is in equilibrium with the surroundings, and no work can be obtained by interactions with the surroundings.

**Diathermal**—heat conducting, and without thermal resistance, but impermeable to mass.

**Efficiency**—see *isentropic efficiency, thermal efficiency, thermodynamic efficiency*.

**Fugacity**—characterizes the escaping tendency of a component, defined mathematically.

**Heteroazeotrope**—mixture that is not completely miscible in all proportions in the liquid phase and like an azeotrope cannot be separated by simple distillation. The heteroazeotropic vapor condenses to two liquid phases, each with a different composition than the vapor. Upon partial or total vaporization, the original vapor composition is reproduced.

**Infinite dilution**—description of a state where a component’s composition approaches zero.

**Irreversible**—a process which generates entropy.

**Isenthalpic**—condition of constant enthalpy.

**Isentropic**—condition of constant entropy.

**Isentropic efficiency**—ratio characterizing actual work relative to ideal work for an isentropic process with the same inlet (or initial) state and the same outlet (or final) pressure. See also *thermodynamic efficiency, thermal efficiency*.

**Isobaric**—condition of constant pressure.

**Isochore**—condition of constant volume. See *isosteric*.

**Isopiestic**—constant or equal pressure.

**Isopycnic**—condition of equal or constant density.

**Isolated**—A system that has no interactions of any kind with the surroundings (e.g. mass, heat, and work interactions) is said to be isolated.

**Isosteric**—condition of constant density. See *isochore*.

**Isothermal**—condition of constant temperature.

**Measurable properties**—variables from the set {*P, V, T, C _{P}, C_{V}*} and derivatives involving only {

**Metastable**—signifies existence of a state which is non-equilibrium, but not unstable, e.g., superheated vapor, subcooled liquid, which may persist until a disturbance creates movement of the system towards equilibrium.

**Nozzle**—a specially designed device which nearly reversibly converts internal energy to kinetic energy. See *throttling*.

**Polytropic exponent**—The exponent *n* in the expression *PV** ^{n}* = constant.

**Quality**—the mass fraction of a vapor/liquid mixture that is vapor.

**rdf**—radical distribution function.

**Reference state**—a state for a pure substance at a specified (*T,P*) and type of phase (*S,L,V*). The reference state is invariant to the system (*P,T*) throughout an entire thermodynamic problem. A problem may have various standard states, but only one reference state. See also *standard state*.

**Sensible heat changes**—heat effects accompanied by a temperature change.

**Specific heat**—another term for *C _{P}* or

**Specific property**—an intensive property per unit mass.

**Solvation**—description of complex formation where the molecules involved are of a different type.

**Spinodal**—condition of instability, beyond which metastability is impossible.

**Standard conditions**—273.15 K and 0.1 MPa (IUPAC), *standard temperature and pressure*.

**Standard state**—a state for a pure substance at a specified (*T,P*) and type of phase (*S,L,V*). The standard state *T* is always at the *T* of interest for a given calculation within a problem. As the *T* of the system changes, the standard state *T* changes. The standard state *P* may be a fixed *P* or may be the *P* of the system. Gibbs energies and chemical potentials are commonly calculated relative to the standard state. For reacting systems, enthalpies and Gibbs energies of formation are commonly tabulated at a fixed pressure of 1 bar and 298.15 K. A temperature correction must be applied to calculate the standard state value at the temperature of interest. A problem may have various standard states, but only one reference state. See also *reference state.*

**State of aggregation**—solid, liquid, or gas.

**Steady-state**—open flow system with no accumulation of mass and where state variables do not change with time inside system boundaries.

**STP**—standard temperature and pressure, 273.15 K and 1 atm. Also referred to as *standard conditions*.

**Subcooled**—description of a state where the temperature is below the saturation temperature for the system pressure, e.g., subcooled vapor is metastable or unstable, subcooled liquid is stable relative to the bubble-point temperature; superheated vapor is stable, superheated liquid is metastable or unstable relative to the dew-point temperature; subcooled liquid is metastable or unstable relative to the fusion temperature.

**Superheated**—description of a state where the temperature is above the saturation temperature for the system pressure. See *subcooled.*

**Thermal efficiency**—the ratio or work obtained to the heat input to a heat engine. No engine may have a higher thermal efficiency than a Carnot engine.

**Thermodynamic efficiency**—ratio characterizing actual work relative to reversible work obtainable for exactly the same change in state variables for a process. The heat transfer for the reversible process will differ from the actual heat transfer. See also *isentropic efficiency, thermal efficiency*.

**Throttling**—a pressure drop without significant change in kinetic energy across a valve, orifice, porous plug, or restriction, which is generally irreversible. See *nozzle*.

**Unstable**—a state that violates thermodynamic stability, and cannot persist. See also *metastable, spinodal.*

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