State is the condition of the system at an instant of time as described or measured by its properties. Or each unique condition of a system is called a state.
It follows from the definition of state that each property has a single value at each state. Stated differently, all properties are state or point functions. Therefore, all properties are identical for identical states.
On the basis of the above discussion, we can determine if a given variable is property or not by applying the following tests :
— A variable is a property, if and only if, it has a single value at each equilibrium state.
— A variable is a property, if and only if, the change in its value between any two prescribed equilibrium states is single-valued.
Therefore, any variable whose change is fixed by the end states is a property.