Which of the following liquid pairs shows a positive deviation from Raoult's law?

Options

A. Water - hydrochloric acid
B. Water-nitric acid
C. Acetone -chloroform
D. Benzene-methonol

Answer

To determine which liquid pair shows a positive deviation from Raoult's law, we need to analyze the interactions between the components of each pair. Here's a step-by-step solution:

### Step 1: Understand Raoult's Law and Deviations
Raoult's law states that the vapor pressure of a solution is directly proportional to the mole fraction of the solvent. Deviations from Raoult's law can be positive or negative, depending on the interactions between the molecules in the solution.

- **Positive Deviation**: Occurs when the vapor pressure of the solution is higher than expected. This happens when the interaction between the solute and solvent (AB) is weaker than the interactions between the solvent molecules (AA) and solute molecules (BB). As a result, the molecules vaporize more easily.

- **Negative Deviation**: Occurs when the vapor pressure of the solution is lower than expected. This happens when the interactions between solute and solvent are stronger than the interactions between the individual solvent and solute molecules.

### Step 2: Analyze Each Option
1. **Water + HCl**: Both are polar molecules, and they exhibit strong dipole-dipole interactions. The interactions will be stronger than the AB interactions, leading to a negative deviation.

2. **Water + Nitric Acid**: Water and nitric acid also form strong hydrogen bonds, which are stronger than the interactions between the individual molecules. This leads to a negative deviation.

3. **Acetone + Chloroform**: Both are polar molecules, and they exhibit dipole-dipole interactions. The interactions will again be stronger than the AB interactions, leading to a negative deviation.

4. **Benzene + Methanol**: Benzene is a non-polar molecule, while methanol is polar and can form hydrogen bonds. The lack of significant interaction between the non-polar benzene and polar methanol means that the molecules can vaporize more easily, leading to a positive deviation.

### Step 3: Conclusion
Among the options analyzed, the pair that shows a positive deviation from Raoult's law is **Benzene + Methanol**.

### Final Answer
The correct answer is **Benzene + Methanol**.

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