PhysChemBasics#1: Change of State 101 Thermodynamics investigates systems – specified regions of the universe. Thermodynamics describes the state of a system using state functions like temperature and volume and also invents new state functions (enthalpy, entropy and free energy) to apply the universal laws of thermodynamics. Thermodynamics call the change of state a "process" and describe it quantitatively using "delta quantities" of state functions and work and heat, which are path functions. As the name implies, path functions are path-dependent, thus the exact path of the process has to be specified. In this lecture, some useful equations are discussed: Gibbs' phase rule to calculate the degrees of freedom of a system, the equation for calculating the sensible heat and the equations for calculating electrical and pressure volume work.