A thermodynamic state is a set of values of properties of a thermodynamic system that must be specified in order to reproduce the system. The individual parameters are known as state variables, state parameters or thermodynamic variables.

When a sufficient set of thermodynamic variables has been specified, the values of all the other properties of the system are unambiguously defined. The number of values needed to specify the state depends on the system, and is not always known.

## State functions

The state functions describe the momentary condition of a thermodynamic system. Regardless of the path that the system follows to go from one state to another, the total change of any state variable will be the same. This means that the incremental changes in these variables are exact differentials. Some examples of a state function are enthalpy, entropy, pressure, temperature, volume, etc.

The state functions of thermodynamic systems generally have a certain interdependence. In the equation of state of an ideal gas, two of the state functions can be arbitrarily selected as independent variables, and other statistical quantities are considered as their functions. The dependence between thermodynamic functions is universal.

There are several thermodynamic diagrams that serve to model the transitions between thermodynamic states.

Each system or type of substance is characterized by a state equation or constitutive equation that relates some of the state variables to each other, since systems in thermodynamic equilibrium have a finite number of degrees of freedom according to the rule of the phases of Gibbs.

## Thermodynamic equilibrium state

The systems found in nature are usually dynamic and complex, but in many cases their states can be described approaching the ideal conditions. One of these ideal conditions is the state of equilibrium. From many observations, thermodynamics postulates that all systems that have no effect on the external environment will tend to change in order to approach the state of equilibrium.

The equilibrium state is a primitive object of classical thermodynamics or equilibrium, in what is called the thermodynamic state. There are several different types of equilibrium, which correspond to different physical variables, and a system reaches the thermodynamic equilibrium when the conditions of all the relevant types of equilibrium are satisfied simultaneously.

Here are some different types of balance.

- Thermal equilibrium: when a system in thermal equilibrium the temperature in a whole system is uniform, the system is in thermal equilibrium.
- Mechanical balance: if at each point within a given system there is no change in pressure over time, and there is no movement of material, the system is in mechanical equilibrium.
- Phase equilibrium: this occurs when the mass for each individual phase reaches a value that does not change over time.
- Chemical equilibrium: in chemical equilibrium, the chemical composition of a system has stabilized and does not change over time.