Delta S of vaporization is defined as heat of vaporization divided by this quantity.

• A. Volume
• B. Pressure
• C. Temperature
• D. Sound

Answer: (C)

Entropy change during a phase change comes from reversibly adding heat and dividing by the temperature at which the change occurs. For vaporization, the heat absorbed per mole reversibly is the heat of vaporization ΔH_vap, and this happens at the boiling temperature where liquid and vapor are in equilibrium. So the entropy change is ΔS_vap = ΔH_vap / T_boil, meaning the quantity in the denominator is temperature. Units line up because ΔH_vap has units of energy per mole and dividing by temperature gives entropy units (J/mol·K). The other quantities—volume, pressure, or sound—do not play the role in this formula, so they aren’t what defines ΔS_vap.