What are state variables?

(N/A) Every equilibrium state of a thermodynamic system is described by specific values of some macroscopic variables called state variables.
An equilibrium state of a gas is specified by the values of pressure,volume,temperature,and mass (and composition if there is a mixture of gases).
$A$ thermodynamic system is not always in equilibrium. $A$ gas allowed to expand freely against a vacuum is not in an equilibrium state.
In a box partition shown in figure $(a)$,a gas is filled in one side and there is no gas on the other side. If the partition is suddenly removed,it leads to the free expansion of the gas,and the pressure of the gas may not be uniform.
In a cylinder as shown in figure $(b)$,a mixture of gases undergoing an explosive chemical reaction is not in an equilibrium state,and here its temperature and pressure are not uniform.
Finally,the gas attains a uniform temperature and pressure and reaches thermal and mechanical equilibrium with its surroundings.