159. Discuss the various types of plots between the partial vapour pressure and the mole fractions of two components of the completely miscible liquids in a solution.

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When the partial vapour pressures of different (two) miscible liquids are plotted against their compositions (mole fractions), following three types of vapour pressure-composition (p – x) curves are obtained.
(a) Type-I: When the vapour pressures of the mixture lie between the vapour pressure of pure components : In such cases the solution obeys the Raoult’s law (ideal solution) i.e., the partial vapour pressure of each component (p_c) is obtained by the relation.

Fig. Solution obeying Raoult’s law.
Where x_c is the mole fraction of that component and p⁰_c the vapour pressure of that component in pure form. In such cases, p-x curve is always a straight line. The total vapour pressure of the solution is equal to the sum of the partial vaour pressures of all components.
Examples: Solution of benzene-toluene, chlorobenzene bromobenzene, hexane-heptane.
(b) Type-II : When the observed vapour pressure of the solution is greater than that of calculated vapour pressure from the Raoult’s law : In such cases partial vapour pressure of each component is found to be more than expected on the basis of the Raoult’s law. The total vapour pressure of the solution is also greater than the vapour pressure corresponding to the ideal solution. At a certain composition the total vapour pressure of the solution will be the highest (maximum) which is greater than the vapour pressure of either of the pure liquids (components) at this components the boiling point of the solution will be lowest. This type of deviation from Raoult’s law is known as positive deviation and the system exhibits a maximum value of vapour pressure at certain composition. At this composition both the liquids boil at same (constant) temperature (minimum boiling azeotropes). In figure point C, corresponds the composition of the two liquids which boils at lowest temperature. For example, alcohol-water mixture having the composition of 95.59% alcohol and 4.41 water boils at 78.13⁰C. This composition is called azeotropic mixture.

Fig.p-x curve showing maximum in the total vapour pressure curve.
Example: Ethanol-water solution. Acetone-carbon disulphide solution. Chloroform-ethanol solution.
(c) Type-III: When the observed vapour pressure of the solution is less than that of calculated from Raoults law: In such cases the partial vapour pressure of any component is found to be less than the expected vapour pressure on the basis of Raoult’s law. Similarly, the total vapour pressure of the solution is also less than that of expected value according to Raoult’s law. At a certain composition the total vapour pressure of such solution will be lowest (minimum). At this composition the boiling point of the solution will be highest (maximum) and both the component will boil at same temperature without the change in the composition. Such composition corresponds to the maximum boiling azeotropic mixture. For example a mixture of 20.24% of HCl and 79.76% water forms an azeotropic mixture which boils at 110⁰C, without the change in composition. In figure, point C, corresponds the composition of azeotropic mixture. This type of deviation in Raoult’s law is known as negative deviation.
Example : Solution of water-HCl, chloroform-benzene, acetone-aniline.

Fig.  p-x curve showing minimum in the total vapour pressure curve.