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

Options

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

Answer

To determine which of the given liquid pairs shows a positive deviation from Raoult's law, we first need to understand what Raoult's law states and what a positive deviation means.

### Step 1: Understand Raoult's Law
Raoult's law states that the partial vapor pressure of each component in a solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution. Mathematically, it can be expressed as:

\[ P_i = X_i \cdot P_i^0 \]

Where:
- \( P_i \) = partial vapor pressure of component i in the solution
- \( X_i \) = mole fraction of component i in the solution
- \( P_i^0 \) = vapor pressure of the pure component i

### Step 2: Define Positive Deviation
A positive deviation from Raoult's law occurs when the total vapor pressure of the solution is greater than the vapor pressure predicted by Raoult's law. This typically happens when the interactions between different molecules in the mixture are weaker than the interactions in the pure components.

### Step 3: Analyze Each Liquid Pair
Now, let's analyze each of the given liquid pairs to see which one shows a positive deviation:

A. **Water and Nitric Acid**: These two have strong hydrogen bonding, which leads to a negative deviation.

B. **Water and Hydrochloric Acid**: Similar to nitric acid, hydrochloric acid also forms strong interactions with water, leading to a negative deviation.

C. **Benzene and Methanol**: Benzene is a non-polar solvent, while methanol is polar. The interactions between benzene and methanol are weaker than the interactions in pure benzene or pure methanol, leading to a positive deviation.

D. **Acetone and Chloroform**: These two solvents can also interact, but the interactions are not as weak as in the case of benzene and methanol. This pair typically shows a negative deviation.

### Step 4: Conclusion
Based on the analysis, the liquid pair that shows a positive deviation from Raoult's law is:

**C. Benzene and Methanol**