ideal Solutions . . Ideal . .
Non-ideal pressure and result in positive deviation. Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them.
Due to weakening of interactions, the solution shows positive deviation from Raoult’s law [Fig. . (a)]. In a solution formed by adding carbon disulphide to acetone, the dipolar interactions between solute-solvent molecules are weaker than the respective interactions among the solute-solute and solvent-solvent molecules.
This solution also shows positive deviation. In case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations. An example of this type is a mixture of phenol and aniline. In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules.
Similarly, a mixture of chloroform and acetone forms a solution with negative deviation from Raoult’s law. This is because chloroform molecule is able to form hydrogen bond with acetone molecule as shown. This decreases the escaping tendency of molecules for each component and consequently the vapour pressure decreases resulting in negative deviation from Raoult’s law [Fig. .
. (b)]. Some liquids on mixing, form azeotropes which are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature. In such cases, it is not possible to separate the components by fractional distillation.
There are two types of azeotropes called minimum boiling azeotrope and maximum boiling azeotrope . The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition. Fig. .
The vapour pressures of two component systems as a function of composition (a) a solution that shows positive deviation from Raoult's law and (b) a solution that shows negative deviation from Raoult's law. For example, ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation gives a solution containing approximately % by volume of ethanol. Once this composition, known as azeotrope composition, has been achieved, the liquid and vapour have the same composition, and no further separation occurs. The solutions that show large negative deviation from Raoult’s law form maximum boiling azeotrope at a specific composition.
Nitric acid and water is an example of this class of azeotrope. This azeotrope has the approximate composition, % nitric acid and % water by mass, with a boiling point of . K.