A system is said to go through a thermodynamic process if its thermodynamic state is altered. We are especially interested in processes with thermodynamic initial and final states while studying equilibrium thermodynamics.

states of equilibrium. Since the process requires removing the system from its initial equilibrium state, the intermediate states cannot all be equilibrium states. This is the result of a gas expanding freely. A gas fills one of the two compartments in a container containing two compartments, while the other is left empty. When the wall between the compartments is removed, the gas expands unrestrictedly, and eventually the balance is reestablished in a situation where the gas