A cylinder fitted with a movable piston contains pure liquid water at 90 °C. The external pressure is slowly reduced while the temperature is held constant at 90 °C, causing a portion of the liquid to vaporize until a new equilibrium is reached.
(a) Determine whether the entropy of the water sample increases, decreases, or remains unchanged after equilibrium is re-established. State your conclusion without calculation.
(b) Provide a particle-level explanation for your answer in part (a), referring specifically to both phase change and volume change.
(c) Predict qualitatively how the entropy change you identified in part (a) would differ if, instead of lowering the external pressure, the temperature were decreased to 80 °C while pressure remained constant at 1 atm. Justify your prediction.