Step 1: Understanding the Concept:
Raoult's law describes the vapor pressure of ideal solutions where interactions between unlike molecules (A-B) are identical to like molecules (A-A or B-B).
Real solutions deviate from this ideality based on the relative strength of intermolecular forces.
Positive deviation occurs when the attractive forces between unlike molecules (A-B) are weaker than those between like molecules.
Because the molecules aren't held as tightly in the mixture, they escape more easily into the vapor phase, resulting in a higher total vapor pressure than predicted.
Negative deviation occurs when the A-B interactions are stronger than the separate components (often due to new hydrogen bond formation).
Step 2: Detailed Explanation:
1. Ethanol + Acetone (Option A): Pure ethanol has very strong intermolecular hydrogen bonding. Acetone molecules are polar but cannot form hydrogen bonds with each other effectively. When mixed, acetone molecules wedge between ethanol molecules and disrupt the ethanol-ethanol hydrogen bonds. This net weakening of intermolecular forces leads to a positive deviation.
2. Benzene + Toluene (Option B): Both are non-polar hydrocarbons with very similar shapes and sizes. They form a nearly ideal solution.
3. Acetone + Chloroform (Option C): These two form a specific, new hydrogen bond between the oxygen of acetone and the hydrogen of chloroform. This new bond makes the A-B interaction stronger than the individual components, leading to negative deviation.
4. Chloroethane + Bromoethane (Option D): These are both alkyl halides with similar polarity and size, making them nearly ideal.
Step 3: Final Answer:
The mixture showing positive deviation is ethanol + acetone.