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Let us first look on the definition

Reversible process: The process where the driving force is only infinitesimally greater than the opposing force is known as Reversible process.

Slow processes may be taken to be Reversible

The reason goes like this

1. Expansion and Contraction of a gas by means of mechanical Work

Let us consider an ideal gas whose co-eff of viscousity = 0. be kept in a vessel (frictionless), with a piston. On the Piston let there be weights, whose gross value is W.

Now at equilibrium if P be the pressure applied by the gas then

W + atm. pressure = P

If the weights are removed suddenly then

P>atm pressure

Hence to reach the equilibrium the gas will expand.

But as the weights are removed suddenly the gas will also expand suddenly and due to this there will be a chaotic movement of gas molecules below the piston, where as some energy will be dissipated to provide the chaotic movement of the molecules, which will not be recoverable.

Thus when the weights are put on the vessel again the gas molecules will not be able to acquire the same amount of Internal energy and the process hence will not be reversible.

But if the weights are removed infinitely slowly then there will be no chaotic movement of the gas molecules and no energy will dissipate. Hence the process will be reversible.

NOTE: In reality co-eff of viscousity is not zero and it is impossible to have a frictionless surroundings Hence some energy will always dissipate against those dissipative forces and processes will always be Irreversible.

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