A thermodynamic system is
said to exist in state of thermodynamic equilibrium when no change in any
macroscopic property is registered, if the system is isolated from its
surroundings.
An isolated system always reaches in course of time a state of thermodynamic
equilibrium and can never depart from its spontaneously.
Therefore, there can be no spontaneous change in any macroscopic property if
the system exists in an equilibrium state.
A thermodynamic system will be in a state of thermodynamic equilibrium, if the
system is the state of Mechanical equilibrium, Chemical equilibrium and Thermal
equilibrium.
1. Mechanical equilibrium: The criteria for Mechanical equilibrium is the equality of pressures.
2. Chemical equilibrium: The criteria for Chemical equilibrium is the equality of chemical potentials.
3. Thermal equilibrium: The criteria for Thermal equilibrium is the equality of temperatures.