A binary liquid solution is prepared by mixing $$n$$-heptane and ethanol. Which one of the following statements is correct regarding the behaviour of the solution?

Raoult’s law states that, for an ideal binary liquid solution, the partial vapour pressure of each component is directly proportional to its mole-fraction in the liquid phase:

$$P_A = X_A P_A^{\,*}, \qquad P_B = X_B P_B^{\,*}$$

where $$P_A^{\,*}$$ and $$P_B^{\,*}$$ are the vapour pressures of the pure liquids $$A$$ and $$B$$. A solution will obey Raoult’s law only when the intermolecular attractions $$A\!-\!A,\,B\!-\!B$$ and $$A\!-\!B$$ are equal in strength.

If unlike-molecule attractions $$A\!-\!B$$ are weaker than either $$A\!-\!A$$ or $$B\!-\!B$$, the molecules escape to vapour more readily, total vapour pressure becomes higher than Raoult’s prediction, and the solution shows a positive deviation from Raoult’s law. Exactly the reverse (stronger $$A\!-\!B$$ attractions) gives a negative deviation.

In the present case the two components are:

• $$n$$-heptane : a non-polar hydrocarbon that relies on weak London dispersion forces.
• Ethanol : a polar molecule capable of strong hydrogen bonding with other ethanol molecules.

When these liquids are mixed, the strong ethanol-ethanol hydrogen bonds are disrupted and replaced partly by much weaker ethanol-heptane interactions. Simultaneously, the weak heptane-heptane attractions are little affected. Therefore

$$\text{(ethanol-heptane attraction)} \lt \text{(ethanol-ethanol attraction)}$$

Overall intermolecular forces in the mixture become weaker than in either pure liquid. Hence molecules of both components tend to escape more easily into the vapour phase, raising the total vapour pressure above the ideal value predicted by Raoult’s law. This is the characteristic signature of a positive deviation.

Consequently, the $$n$$-heptane + ethanol system is a non-ideal solution that exhibits positive deviation from Raoult’s law.

Option B which is: The solution is non-ideal, showing +ve deviation from Raoult’s law.