# Which one is the correct definition of state function or state variable?

Three definitions for state function or state variable: "Properties which define state of any system are called it's state variables".

"Properties of the system which depend only upon initial and final state is the system but does not depend upon the path or mechanism followed bt the system to achieve the final state are called state functions".

"Any property of the system which defines the state of the system, but is independant of the path that brings about the change in state is known an state function". Which definition is correct for state function which definition is correct for state variable?

Also, could you please explain what is the difference between state function and state variable in thermodynamics?

• The state functions and state varibales can play interchanged roles. For example, the entropy $S$ can be treated as an state function, $S = S(U, N, V)$. But it can be treated as a state varibale $U = U(S, V, N)$.
– ytlu
Apr 16, 2021 at 11:11
• All three definitions are correct. A (thermodynamic equilibrium) state function and a (thermodynamic equilibrium) state variable are the same. The word "state" implies thermodynamic equilibrium. The state of a pure single phase system of constant composition is specified once any two state variables are specified. This determines all the others. Apr 16, 2021 at 11:21