An irreversible process is a thermodynamic process that cannot return both the system and the surroundings to their original states without leaving a net change in the universe. This is often due to factors like friction, unrestrained expansion, mixing of different substances, heat transfer through a finite temperature difference, and other dissipative effects.
Processes are considered irreversible because they involve the generation of entropy, which is a measure of disorder or randomness in a system. Once entropy is generated, it cannot be completely eliminated, making it impossible to return the system to its exact initial state without external intervention. This generation of entropy often manifests as heat loss, frictional forces, or other forms of energy dissipation.
In theory, an irreversible process cannot become reversible even given infinite time. This is because the fundamental nature of irreversibility involves the creation of entropy, which cannot be undone. While certain aspects of the system might approach a state of equilibrium over infinite time, the overall process remains irreversible due to the net increase in entropy.
Entropy is a central concept in determining the irreversibility of processes. In any irreversible process, the total entropy of the system and its surroundings increases. This increase in entropy is a measure of the energy that is no longer available to do useful work. Since entropy cannot decrease in an isolated system, it ensures that the process cannot be reversed without external intervention that would further increase the entropy of the surroundings.
In practical terms, there are no exceptions to the irreversibility of processes as dictated by the second law of thermodynamics. However, in theoretical constructs like reversible processes (idealized processes where entropy generation is zero), a process can be reversed. These reversible processes are idealizations and do not occur in the real world, where all natural processes involve some degree of irreversibility due to entropy production.