Theory of partially miscible liquid pairs

Problem 16 Discuss the theory of partially miscible liquid pairs with special reference to  

Define the upper and lower critical solution temperature. What is the effect of impurities on them (Meerut 2004) 

If we consider two liquids A and B and shake together, then some of A dissolves in B, while some of B dissolves in A. We then have two saturated solutions—one of A in B and the other of B in A. On increasing the temperature, the solubility of A increases in B and also that of B in A, in this particular case. As an example, we take the familiar phenol-water system. 

Phenol and water are partially miscible at ordinary temperature. When we add phenol to water phenol gets dissolved in water, till its concentration reaches 8%. The addition of more phenol will give rise to two liquid layers. One layer will consist of water in phenol system and the other that of phenol in water system. Such solutions of different compositions co-existing in equilibrium with one another are known as conjugate solutions. 

As the temperature is raised, the solubility of phenol in water increases, whereas that of water in phenol also increases. Ultimately at a certain temperature, the two conjugate solutions change into one homogeneous solution. This temperature is known as critical solution temperature or consolute temperature. The value of consolute temperature for this system is 68.3°, and the composition is 33% phenol and 67% water. Above 68.3°, the two liquids are completely miscible in all proportions. The variation of mutual solubility of water and phenol with temperature is shown in fig. (16). The solubility of phenol in water increases with rise of temperature along die curve AB, while the solubility of water in phenol increases along CB. The two curves do not intersect each other, but meet at a certain point B, known as C.S.T. 

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