The enthalpy of a binary mixture of
is plotted as a function of the mole fraction of component
. For an ideal solution, the enthalpy of the mixture is a linear function of the molar enthalpies of the pure components. For a non-ideal solution, you can vary a parameter that describes the deviation from ideality. One can define partial molar enthalpies, which are obtained by drawing a tangent at the value of the mole fraction of the solution. The intersections of this tangent with the
correspond to the partial molar enthalpies of
, respectively. You can also vary the mole fraction of
in the mixture, as indicated by the dot on the curve.