Step 1: Understanding the Concept:
According to Raoult's law, non-ideal solutions show either positive or negative deviation.
A positive deviation occurs when the intermolecular attractive forces between dissimilar molecules (A-B interactions) are weaker than those between similar molecules (A-A and B-B interactions).
This causes the molecules to escape more easily into the vapor phase, resulting in a higher vapor pressure than predicted by Raoult's law.
Step 2: Key Formula or Approach:
The approach is to compare the strength of intermolecular forces in the pure components versus the mixture.
If new A-B interactions are weaker than A-A and B-B interactions, the mixture exhibits a positive deviation.
Step 3: Detailed Explanation:
Let us analyze the intermolecular forces in each given mixture:
(B) Benzene and toluene: Both are non-polar hydrocarbons with similar structures. Their intermolecular forces (London dispersion forces) are nearly identical in pure state and in the mixture. They form an almost ideal solution.
(C) Chloroform and acetone: Pure acetone has dipole-dipole interactions, and pure chloroform also has dipole-dipole interactions. When mixed, they form strong intermolecular hydrogen bonds between the hydrogen of chloroform and the oxygen of acetone.
Since new A-B interactions are stronger than A-A or B-B, it exhibits negative deviation.
(D) Phenol and aniline: Similar to the previous case, strong intermolecular hydrogen bonding occurs between the phenolic proton and the lone pair of nitrogen in aniline. This leads to a negative deviation.
(A) Ethanol and acetone: In pure ethanol, molecules are held together tightly by strong extensive hydrogen bonding.
When acetone is added, its molecules interpose themselves between the ethanol molecules, breaking some of the existing hydrogen bonds.
Consequently, the attractive forces in the mixture (ethanol-acetone) become weaker than the strong hydrogen bonds in pure ethanol.
This makes it easier for molecules to escape, leading to a higher vapor pressure and a positive deviation from Raoult's law.
Step 4: Final Answer:
The mixture of ethanol and acetone exhibits a positive deviation.