Reversible vs. Irreversible Processes
A change is said to occur reversibly when it can be carried out in a series of infinitesimal steps. Each of the steps can be undone by making a similarly minute change to the conditions that bring about the change. For example:
- The reversible expansion of a gas can be achieved by reducing the external pressure in a series of infinitesimal steps; reversing any step will restore the system and its surroundings to their previous state.
- Similarly, heat can be transferred reversibly between two bodies by changing the temperature difference between them in infinitesimal steps, each of which can be undone by reversing the temperature difference.
The most widely cited example of an irreversible change is the expansion of a gas into a vacuum. Although the system can always be restored to its original state by recompressing the gas, this would require that the surroundings perform work on the gas. Since the gas does no work on the surroundings in a free expansion (the external pressure is zero, so PΔV = 0,), there will be a permanent change in the surroundings. Another example of an irreversible change is the conversion of mechanical work into frictional heat; there is no way, by reversing the motion of a weight along a surface, that the heat released due to friction can be restored to the system.
Consider a glass of ice water in a room. The heat from the surroundings (entropy) goes into the ice water and the ice melts. The entropy of the ice water increases while the entropy of the surroundings decreases.