Reversible process in Thermodynamics
From thermodynamical point of view, a process is said to be reversible, if it can be retraced in the opposite direction at any stage of the operation in such a manner that the system and the surroundings pass through exactly the same state, at each stage, as in the direct process. After the end of the process, the system and the surroundings taking part in the process are restored to their initial state without producing change in either of them.
Conditions for a process to be reversible
1. All steps involved in the process must occur infinitely slowly.
2. Dissipative forces such as friction, viscosity, electrical resistance etc must be completely absent.
3. The pressure and temperature of the system must never differ appreciably from surroundings at any stage of the process.
No actual process is fully reversible; but many processes when carried out slowly are practically reversible. For example, the slow compression of a spring is practically reversible, because if the force is slowly decreased it expands and performs the work, equal to the work done in compressing it. The slow evaporation of a substance in an insulated container is practically reversible.
Melting and boiling processes are reversible. In ideal case isothermal and adiabatic processes are reversible.
Electrolysis is a reversible process if the electrical resistance of the electrolyte is zero. On reversing the direction of current, the direction of motion of ions is reversed.
The Peltier effect is a reversible process.
Irreversible process
Any process which cannot be retraced in the opposite direction by reversing the controlling factors is called irreversible process. It is obvious that in such a process, changes occur suddenly and it will be accompanied by dissipative forces.
For example, motion of a body on the ground is irreversible because energy spent in overcoming friction is not recoverable. Passage of electric current through a resistor is irreversible because heat is evolved whether current passes in one direction or in the opposite. All chemical reactions are irreversible because they involve change in the internal structure of the constituents.
Water fall is an unreversible process. The rusting of iron is an irreversible process.