Entropy changes in isothermal transformations

For any isothermal change in state, T, being constant, can be removed from the integral in Eq. (9.2), which then reduces immediately to 

The entropy change for the transformation can be calculated by evaluating the quantities of heat required to conduct the change in state reversibly. 

By the same argument, the entropy of fusion at the melting point is given by 

For many liquids, the entropy of vaporization at the normal boiling point has approximately the same value  

Equation (9.8) is Trouton s rule. It follows immediately that for liquids which obey this rule, 

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