The binary solution of miscible liquids, when heated from liquid state at a fixed pressure, a stage comes when we have first bubble formation, this temperature is bubble point of the solution. At this stage the total pressure is equal to the summation of its partial pressure and for an ideal system in vapor-liquid equilibrium:
P = x1P1 + x2P2
at bubble point the liquid phase has the same composition as original solution, since there is only a single bubble formation. As we increase the temperature above bubble point, the boiling continues leads to decrease in concentration of more volatile component in liquid phase as well as in vapor phase. The vapor pressure increases as temperature increases but the composition decreases so as to maintain the constant pressure. At a point all the liquid has been vaporized and we are left with last drop of liquid in equilibrium with vapor phase called Dew point. at this point, the vapor phase composition is same as original liquid mixture composition.
The solution start boiling at its bubble point and stops at its dew point. The boiling temperature of the solution is between the boiling temperature of two components.
The im-miscible liquids start boiling at a temperature lower than the boiling point of either component. because each component contribute to the total pressure as it is pure. Therefore, when the sum of vapor pressure becomes equal to total pressure, the solution start boiling. This is used in steam distillation, where we use steam to distill heat sensitive components.
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