text: Physical systems found in nature are practically always dynamic and complex, but in many cases, macroscopic physical systems are amenable to description based on proximity to ideal conditions.  One such ideal condition is that of a stable equilibrium state.  Such a state is a primitive object of classical or equilibrium thermodynamics, in which it is called a thermodynamic state. Based on many observations, thermodynamics postulates that all systems that are isolated from the external environment will evolve so as to approach unique stable equilibrium states. There are a number of different types of equilibrium, corresponding to different physical variables, and a system reaches thermodynamic equilibrium when the conditions of all the relevant types of equilibrium are simultaneously satisfied. A few different types of equilibrium are listed below.
  *Thermal equilibrium: When the temperature throughout a system is uniform, the system is in thermal equilibrium.
  *Mechanical equilibrium: If at every point within a given system there is no change in pressure with 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 with time. 
  *Chemical equilibrium: In chemical equilibrium, the chemical composition of a system has settled and does not change with time.