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Q.An azeotropic solution of two liquids has boiling point lower than either of them when it : (A) is saturated (B) shows positive deviation from Raoult's law (C) shows negative deviation from Raoult's law (D) shows no deviation from Raoult's law

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An azeotrope with a boiling point lower than either pure component forms when the mixture shows positive deviation from Raoult's law, because the components "want to escape" the liquid phase more readily than ideal behavior predicts. The answer is (B).

Understanding Azeotropes and Deviations from Ideality

An azeotrope is a constant-boiling mixture where the vapor has the same composition as the liquid. This means you cannot separate the components by simple distillation at that composition. The key to understanding which type of azeotrope forms lies in how the solution deviates from Raoult's law.

Raoult's law for an ideal solution states that the partial vapor pressure of each component is proportional to its mole fraction:

Pi=xiPi0P_i = x_i P_i^0

where Pi0P_i^0 is the vapor pressure of the pure component. Real solutions often deviate from this ideal behavior due to intermolecular forces between unlike molecules differing from those between like molecules.

The Connection Between Deviation and Boiling Point

The boiling point of a liquid is reached when its total vapor pressure equals atmospheric pressure. If a mixture has a higher total vapor pressure than predicted by Raoult's law, it will boil at a lower temperature. Conversely, if the vapor pressure is lower than ideal, the boiling point rises.

Step-by-Step Analysis

  1. Positive Deviation from Raoult's Law

    When AA-BB interactions are weaker than AA-AA and BB-BB interactions, molecules escape the liquid phase more easily than in an ideal solution. The total vapor pressure is:

Ptotal>xAPA0+xBPB0P_{\text{total}} > x_A P_A^0 + x_B P_B^0

This increased vapor pressure means the mixture reaches atmospheric pressure at a lower temperature than either pure component. The azeotrope formed is a minimum-boiling azeotrope.

  1. Negative Deviation from Raoult's Law

    When AA-BB interactions are stronger than AA-AA and BB-BB interactions (hydrogen bonding, for example), molecules are held more tightly in the liquid phase:

Ptotal<xAPA0+xBPB0P_{\text{total}} < x_A P_A^0 + x_B P_B^0

The reduced vapor pressure means a higher boiling point than either pure component. This forms a maximum-boiling azeotrope.

  1. No Deviation (Ideal Solution)

    An ideal solution follows Raoult's law exactly. No azeotrope forms because the vapor composition continuously changes with liquid composition during distillation. Complete separation is possible.

  2. Saturation …

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