We see so many changes happening around us every day, such as boiling water, rusting of iron, melting ice, burning of paper, etc. In all these processes, we observe that the system in consideration goes from an initial state \(i\) to a final state \(f\). During the process, the system absorbs heat \(Q\) from the surroundings and performs work \(W\) on it. Can we reverse this process and bring both the system and surroundings to their initial states with no other effect anywhere? 

What Are Reversible Processes?

A thermodynamic process (state \(i \rightarrow\) state \(f\) ) is reversible if the process can be turned back such that both the system and the surroundings return to their original states, with no other change anywhere else in the universe. A reversible process is an idealised notion. In reality, no such processes as reversible processes can exist. A process is reversible only if it is quasi-static (system in equilibrium with the surroundings at every stage) and there are no dissipative effects. For example, a quasi-static isothermal expansion of an ideal gas in a cylinder fitted with a frictionless movable piston is a reversible process.

Here, we have listed a few examples of Reversible Processes:

• Extension of springs
• Slow adiabatic compression or expansion of gases.
• The frictionless motion of solids.
• Slow isothermal compression or expansion of gases.

What Are Irreversible Processes?

An irreversible process can be defined as a process in which the system and the surroundings do not return to their original condition once the process is initiated. Take an example of an automobile engine that has travelled a distance with the aid of fuel. During the process, the fuel burns to provide energy to the engine, converting itself into smoke and heat energy. We cannot retrieve the energy lost by the fuel and cannot get back the original form. Thus, some processes are reversible while others are irreversible in nature, depending upon their ability to return to their original state from their final state.

A few examples of Irreversible Processes are:

• Relative motion with friction
• Heat transfer
• Diffusion
• Electricity flows through a resistance

Entropy

Like pressure, volume, temperature, internal energy, etc., we have another thermodynamic variable of a system, named entropy. In a given equilibrium state, the system has a definite value of entropy. If the system has a temperature \(T\) (in absolute scale) and a small amount of heat \(\Delta Q\) is given to it, we define the change in the entropy of the system as
\(
\Delta S=\frac{\Delta Q}{T}
\)
In general, the temperature of the system may change during a process. If the process is reversible, the change in entropy is defined as
\(
S_f-S_i=\int_i^f \frac{\Delta Q}{T} .
\)
In an adiabatic reversible process, no heat is given to the system. The entropy of the system remains constant in such a process.

Entropy is related to the disorder in the system. Thus, if all the molecules in a given sample of a gas are made to move in the same direction with the same velocity, the entropy will be smaller than that in the actual situation in which the molecules move randomly in all directions.

An interesting fact about entropy is that it is not a conserved quantity. More interesting is the fact that entropy can be created but cannot be destroyed. Once some entropy is created in a process, the universe has to carry the burden of that entropy for ever. The second law of thermodynamics may be stated in terms of entropy as follows:

It is not possible to have a process in which the entropy of an isolated system is decreased.