Why Ideal Solutions

We will discuss the estimation of the fugacities of species in mixtures in Section 7.12 after we have introduced some more useful definitions. 

It follows from Eq. 7.21 that the integral in Eq. 7.18. is zero, or that 

Comparing these equations with the corresponding ones for pure species, we see that the pressure P has been replaced by the partial pressure Pxi and the volume, enthalpy, compressibility factor, and volume residual have been replaced by their partial molar equivalents. Here we introduce the 

as shown above, for ideal gas mixtures the fugacity of one species in the mixture is equal to its partial pressure, then we would like to extend that simple idea to nonideal gas mixtures, and to solutions of liquids and solids. We can, using the definition of an ideal solution. An ideal solution is like an ideal gas in the following respects  

Neither exists exactly in nature. There are no gases that show exactly ideal gas behavior over a wide range of pressures, and no solutions that show exactly ideal solution behavior over a wide range of compositions. 

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