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Immiscible liquids: Topic for CSIR-UGC-NET, GATE and IIT-JAM Exams

Immiscible Liquids:

Immiscible liquids are liquids that do not mix to give a homogenous solution. Liquid immiscibility is a state in which two liquids with different compositions coexist in equilibrium with each other. Immiscible liquids do not mix and form an emulsion of droplets or networks of one liquid within the other (e.g. oil and water). Liquids can become immiscible on lowering the temperature and remain unmixed into two liquids. Immiscibility occurs when the Gibbs free energy of the two liquids is lesser than the gibbs free energy of a mixed liquids. 



In chemistry, when two liquids mix together, they can be classified as miscible or immiscible. In miscible liquids, the bonds that are holding each separate liquid together must be broken and new bonds must be formed. When this occurs, the two liquids mix together in equal proportions and mix completely. Immiscible liquids are the opposite. Their bonds are not broken in order to form new bonds, and the two liquids will not form one cohesive solution. An example of an immiscible liquid solution is oil and water.

Consider the distillation of immiscible liquids, such as mixture of octane and water. At equilibrium, there is a negligible amount of A dissolved in B, it is called saturated solution of A in B. Similarly, a negligible amount of B is dissolved in A, it is called saturated solution of B in A. Both liquids are saturated with the other component. The total vapour pressure of the mixture is close to p=pA +pB. If the temperature of the mixture is raised to a value at which this total vapour pressure is equal to a pressure equal to the atmospheric pressure, boiling is started and the dissolved substances are purged from their solution. The presence of the saturated solutions means that the ‘mixture’ boils at a lower temperature than either component would alone because boiling begins when the total vapour pressure becomes equal to one atmospheic pressure, not when individual vapour pressure(either pA or pB) reaches one atmospheic pressure. This is the basis of steam distillation. Water-insoluble organic compounds can be distilled off at a lower temperature than their normal boiling point. The composition of the condensate is same as the composition of vapour phase of both the components. The oils having low volatility (high boiling temperature) distil in low amount as compare to high volatile water (having low boiling temperature).

Immiscible liquids is a very important topic for the study of solution asked in different exams like CSIR-UGC-NET/JRF, gate and IIT-JAM and different prestigious exams and give a very good contribution for cracking these exams..

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