The entropy is a state variable, so the entropy change should not depend on the path between the initial and final state. That's why we can calculate the entropy change of free expansion with an isothermal process. In chapter 20 of Halliday's Fundamental of Physics, it says that ________. If a process occurs in a closed system, the entropy of the system increases for irreversible processes and remains constant for reversible processes. It never decreases. It seems like the entropy change depends on whether the process is reversible or not. Why doesn't the statement violate the assumption of 'entropy is a state variable'?