Chemistry

In a reversible isothermal expansion, the gas absorbs heat $q$ from the surroundings and its entropy increases by $\Delta S_{\text{gas}} = q/T$. The surroundings lose that same heat and their entropy decreases by exactly $\Delta S_{\text{surr}} = -q/T$, so the two changes cancel perfectly—this perfect balance is what defines a reversible process and why $\Delta S_{\text{universe}} = 0$.

In a reversible isothermal expansion, the gas absorbs heat $q$ from the surroundings and its entropy increases by $\Delta S_{\text{gas}} = q/T$. The surroundings lose that same heat and their entropy decreases by exactly $\Delta S_{\text{surr}} = -q/T$, so the two changes cancel perfectly—this perfect balance is what defines a reversible process and why $\Delta S_{\text{universe}} = 0$.