Reversible and Irreversible Processes

Consider a hot cup of coffee losing its heat to the surroundings, or the mixing of two different gases inside a container when the partition is removed. There is no way to reverse these processes naturally.

These thermodynamic processes, in which the system and surroundings together cannot be restored to their original state, are called irreversible processes.

The irreversibility of any natural process results from the second law of thermodynamics. According to Clausius's statement, "heat never flows spontaneously from a colder body to a hotter body."

However, we can think of some ideal processes that are reversible, but they can never occur naturally.

For example, consider a gradual compression of the gas or expansion of the gas by changing the weight on the piston by a tiny amount.

In this example, the process path is traced back to restore the system and the surroundings to their initial state.

Since the process takes place in infinitesimally small steps, the system is in equilibrium with the surroundings, and the process can be approximated as a reversible process