Question

Which of the following solutions shows positive deviation from Raoult's law?

• A. Ethanol + Acetone
• B. Chloroform + Acetone
• C. Benzene + Toluene
• D. Phenol + Aniline

Hint:

If mixing two liquids breaks existing hydrogen bonds (like adding a non-polar or less polar solvent to an alcohol), expect a positive deviation. If mixing them creates new hydrogen bonds (like mixing an acid and a base), expect a negative deviation!

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
We must identify which of the given liquid-liquid mixtures exhibits a positive deviation from Raoult's Law.

Step 2: Key Formula or Approach:
A solution shows a positive deviation from Raoult's Law when the solute-solvent intermolecular interactions (A-B) are weaker than the solute-solute (A-A) and solvent-solvent (B-B) interactions.
Because the new interactions are weaker, the molecules can escape more easily into the vapor phase, resulting in a higher vapor pressure than predicted by Raoult's Law.

Step 3: Detailed Explanation:
Let's analyze the mixtures:
(A)

Ethanol + Acetone: In pure ethanol, the molecules are strongly held together by extensive intermolecular hydrogen bonding. When acetone is added, its molecules interpose themselves between the ethanol molecules, breaking some of these hydrogen bonds. The resulting A-B interactions are weaker than the original A-A interactions, leading to a positive deviation.
(B)

Chloroform + Acetone: These two molecules can form a new intermolecular hydrogen bond with each other that didn't exist in the pure liquids. This makes A-B interactions stronger, leading to a negative deviation.
(C)

Benzene + Toluene: These are structurally very similar non-polar hydrocarbons. The A-B interactions are almost identical to A-A and B-B interactions, so this forms an ideal solution (no deviation).
(D)

Phenol + Aniline: These molecules form strong intermolecular hydrogen bonds with each other (acid-base type interaction), resulting in stronger A-B forces and a negative deviation.

Step 4: Final Answer:
Ethanol + Acetone shows a positive deviation, which matches option (A).