Question

Which from following mixtures exhibits positive deviation from Raoult's law?

• A. Ethanol and acetone
• B. Benzene and toluene
• C. Chloroform and acetone
• D. Phenol and aniline

Hint:

Positive deviation occurs when mixing breaks strong existing interactions and makes molecules escape more easily into vapour phase.

Answer 1

The Correct Option is A

Concept:
A solution shows positive deviation from Raoult's law when:
• attraction between unlike molecules is weaker than attraction between like molecules,
• vapour pressure becomes higher than expected.

Step 1: Check benzene and toluene.
Benzene and toluene are chemically similar and behave nearly ideally. So they do not show significant positive deviation.

Step 2: Check chloroform and acetone, phenol and aniline.
These mixtures form strong intermolecular attractions such as hydrogen bonding. So they show negative deviation, not positive deviation.

Step 3: Check ethanol and acetone.
When ethanol is mixed with acetone, the strong hydrogen bonding between ethanol molecules gets weakened. So the attraction between unlike molecules becomes weaker than in pure ethanol. Therefore, this mixture shows positive deviation from Raoult's law. Hence, the correct answer is:
\[ \boxed{(A)\ \text{Ethanol and acetone}} \]