Solid Equilibrium at Constant Pressure

In this section, we consider a system consisting of two substances which are immiscible in the solid state but which are miscible throughout the entire composition range in the liquid phase. The temperature-composition diagram of this system is represented in Fig. 9-10. Phase 2 is pure solid 1 or pure solid 2, while phase 1 is the liquid. 

We first consider the equilibrium between the hquid solution and pure solid 1 in this case, y, = 1 and yj = 0. Under these conditions, Eq. (9-57) becomes 

If Lj is positive, the solubility of component 2 decreases with increasing temperature. 2 is often called the last heat of solution, i.e., the differential heat of solution into saturated solution, and for most electrolytes is negative. 

If we approximate L2(T,p) by LiiTitP), where T2 is the melting point of pure solid 2 at pressure p, Eq. (9-111) can be easily integrated to yield 

T - T Xi) is the temperature at which pure solid 2 is in equilibrium with the liquid solution with mole fraction X2 at pressure p. Thus, T is the freezing point of the liquid solution with mole fraction X2. Equation (9-113) is the equation for curve B in Fig. 9-10. Similarly, from Eq. (9-106), we obtain the equation for curve A in Fig. 9-10 in the form 

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