Question

Which of the following solutions behaves nearly as an ideal solution?

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

Hint:

To spot an ideal solution instantly on an exam, look for pairs that belong to the same homologous series or have nearly identical structures! Classic examples include Benzene + Toluene, $n$-Hexane + $n$-Heptane, and Bromoethane + Chloroethane.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks us to identify which of the given liquid pairs forms a solution that behaves nearly ideally over the entire concentration range.

Step 2: Key Formula or Approach:
An ideal solution is one that strictly obeys Raoult's Law at all temperatures and concentrations. For a solution to be ideal, the intermolecular attractive forces between the solute and solvent molecules ($A-B$ interactions) must be nearly identical in magnitude to those within the pure components ($A-A$ and $B-B$ interactions). This typically occurs when the two components have very similar chemical structures, polarities, and molecular sizes.

Step 3: Detailed Explanation:
Let us analyze the nature of the molecular interactions in each pair:
(A) Benzene and Toluene: Both are non-polar aromatic hydrocarbons of comparable molecular shape and size. The London dispersion forces between benzene-benzene, toluene-toluene, and benzene-toluene are nearly identical. Therefore, their mixture satisfies the conditions $\Delta H_{\text{mix}} \approx 0$ and $\Delta V_{\text{mix}} \approx 0$, forming a nearly ideal solution.
(B) Phenol + aniline: This pair shows a strong negative deviation from Raoult's law because the intermolecular hydrogen bonding between phenolic hydrogen and aniline nitrogen ($A-B$) is stronger than the interactions in the pure liquids.
(C) Chloroform + acetone: This mixture also displays a negative deviation due to the formation of a strong intermolecular hydrogen bond between the acidic chloroform hydrogen and the basic carbonyl oxygen of acetone.
(D) Ethanol + acetone: This pair exhibits a positive deviation from Raoult's law because adding acetone molecules disrupts the highly organized hydrogen-bonded network of pure ethanol, making it easier for components to escape into the vapor phase.

Step 4: Final Answer:
The solution that behaves nearly as an ideal solution is Benzene + toluene, which corresponds to option (A).