Question

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

Answer 1

Hint: Azeotropic solution is the solution which contains two liquids and has constant vapour and liquid composition. The azeotropic solutions cannot be separated with the help of distillation process. They are divided into two parts i.e. positive and negative deviation.

Complete answer:
-Roult's law states that the vapour pressure of a solution which contains non-volatile solute is directly proportional to the mole fraction of the solvent.
-Now, we know that there are two types of deviation according to the Roult's Law i.e. Positive and Negative deviation.
-In positive deviation, the partial pressure of each component that is present in the solution is greater than the vapour pressure as expected or they have lower boiling point than the expected value.
-In this type of deviation, the bond between solute-solvent is weaker than the bond present between solute-solute and solvent-solvent.
-So, it is easy for them to break the bonds at lower temperature and hence, have lower boiling point.
-The azeotropic solutions which show positive deviation are known as minimum boiling azeotrope.
-Whereas the solution which obeys Roult's law are known as ideal solutions and do not show any deviation in the vapour pressure and boiling point.
So, the correct answer is “Option A”.

Note: Negative deviations are those in which the partial vapour pressure of each component of solution is less than the expected value of it. For example, the solution of acetone and chloroform. The bond between solute-solvent is stronger than the bond present between solute-solute and solvent-solvent.