**Derivation from state postulate**
Using the state postulate, take the specific entropy, for a homogeneous substance to be a function of specific volume and temperature .[3]:508

During a phase change, the temperature is constant, so[3]:508

Using the appropriate Maxwell relation gives[3]:508

Since temperature and pressure are constant during a phase transition, the derivative of pressure with respect to temperature is not a function of the specific volume.[4][5]:57, 62 & 671 Thus the partial derivative may be changed into a total derivative and be factored out when taking an integral from one phase to another,[3]:508

Here and are respectively the change in specific entropy and specific volume from the initial phase to the final phase .

For a closed system undergoing an internally reversible process, the first law is

Using the definition of specific enthalpy, and the fact that the temperature and pressure are constant, we have[3]:508

After substitution of this result into the derivative of the pressure, one finds[3]:508[6]

This last equation is the Clapeyron equation.

Source:

http://en.wikipedia.org/wiki/Clausiu...eyron_relation
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