I would like someone to explain to me the correlations between these thermodynamics concepts: 1 State function/conservative force/reversibility 2 State function/exact differential Some functions in the phase space of a system are state function some are not. Is this simply an empirical fact or is there something deeper about it? AFAIK, heat, for instance, is an inexact differential, but it is also given by the mass times the specific heat times the variation in temperature so it is also expressible as an exact differential. How can it be? Is the concept of state function only meaningful in a state of equilibrium? What is the role of time in thermodynamics? Why we always write equation without any reference to time? Is it possible to write an equation of evolution of a thermodynamics system just like the laws of motion in classical mechanics?