A thermodynamic state is a set of values of properties of a thermodynamic system that must be specified to reproduce the system. The individual parameters are known as state variables, state parameters or thermodynamic variables. Once a sufficient set of thermodynamic variables have been specified, values of all other properties of the system are uniquely determined. The number of values required to specify the state depends on the system, and is not always known.
State Variable:
Thermodynamic variables describe the momentary condition of a thermodynamic system. Regardless of the path by which a system goes from one state to another — i.e., the sequence of intermediate states — the total changes in any state variable will be the same. This means that the incremental changes in such variables are exact differentials. Examples of state variables include:
Density (ρ)
Energy (E)
Helmholtz free energy (A)
Gibbs free energy (G)
Enthalpy (H)
Internal energy (U)
Mass (m)
Energy
Pressure (p)
Entropy (S)
Temperature (T)
Volume (V)
Equilibrium state:
Systems found in nature are often dynamic and complex, but in many cases their states are amenable to description based on proximity to ideal conditions. One such ideal condition is that of a stable equilibrium state. Based on many observations, thermodynamics postulates that all systems having no effect on the external environment will change in such a way as to approach unique stable equilibrium states.
For Example:
A common example in which the state can be succinctly described is a closed simple system in an equilibrium state. A closed simple system is an ideal system devoid of any internal adiabatic, rigid, or impermeable boundaries and not being acted upon by any external force fields or inertial forces. Based on observation, scientists and engineers have postulated that the state of a simple system at equilibrium can be completely characterized by specifying two independent property variables, such as temperature and pressure, and the masses of the particular chemical species in the system. Relying on this postulate, for many chemical species, phase distribution and intrinsic phase properties such as density, heat capacity, thermal conductivity, viscosity, enthalpy, and entropy have been reproducibly measured and cataloged as functions of temperature and pressure.
