In the paper, linear regressions of 𝑅 as the condensation entropy, it is observed that the entropy is about half of the entropy derived from Gibbs's free energy, i.e., ∆G = 0. This result indicates that Gibbs's theorem is disputable, namely that the reversibility is impossible according to the second law of thermodynamics. Indeed, the exothermic heat of liquefaction is not equal to the endothermal heat of evaporation. Clausius-Clapeyron equation is derived from the exothermic heat equations. Helium-4 superfluid abnormally exhibits a continual entropy increase in liquid as T decreases from 3.5 to 0.8 K. A similar finding in the linear regression curve of H2 as the temperature approaches 14 K, proposes that the entropy increase in liquid is a common phenomenon as the temperature approaches 0 K.