Classical thermodynamics studies the equilibrium properties of systems, the processes being reversible. Now we are going to study systems that are out of equilibrium, in which irreversible processes occur. A system is out of equilibrium because matter or energy is transported between the system and the surroundings, or between different parts of the system. These out-of-equilibrium systems evolve, over time, following irreversible processes towards equilibrium states.
Two examples of irreversible processes are the dissolution of a salt and the transfer of heat between two heat sources at different temperatures.