Heat and work is a transfer process, and how much heat and work is generated refers to the change from one state to another, but the cumulative amount of change between States is not a certain state value, so heat and work are not state functions.

The state function, that is, the macroscopic properties that characterize the characteristics of the system, refers to thermodynamic functions with energy dimensions (such as internal energy, enthalpy, Gibbs free energy and Helmholtz free energy). The state function only has a definite value for the system in equilibrium state, and its change value only depends on the initial state and final state of the system. In addition, the functions of the state are interrelated and mutually restricted. According to its nature, the state function can be divided into two categories, namely, the breadth nature and the intensity nature, and the difference lies in whether it is related to the quantity of matter.