Which of the following is correct about a solution showing positive deviation?

Options

A. Vapour pressure observed will be less than calculated from Raoult's law
B. Minimum boiling azeotrope will be formed
C. `Delta H_(mix) lt 0`
D. `Delta V_(mix) lt 0`

Answer

To determine which of the statements is correct about a solution showing positive deviation from Raoult's law, we need to analyze the characteristics of such solutions step by step.

### Step 1: Understanding Positive Deviation
A solution exhibits positive deviation from Raoult's law when the vapor pressure of the solution is greater than what would be predicted by Raoult's law. This occurs when the intermolecular forces between the different components of the solution are weaker than those in the pure components.

**Hint:** Recall that Raoult's law states that the vapor pressure of a solution is dependent on the vapor pressures of the individual components and their mole fractions.

### Step 2: Analyzing Vapor Pressure
In a solution showing positive deviation, the total vapor pressure observed is greater than the vapor pressure calculated using Raoult's law. Therefore, if we have a statement claiming that the vapor pressure observed is less than calculated from Raoult's law, that statement is incorrect.

**Hint:** Think about how the intermolecular forces affect the vapor pressure of the solution.

### Step 3: Understanding ΔV and ΔH
For solutions showing positive deviation:
- **ΔV (change in volume)** is greater than 0. This indicates that the volume of the solution is greater than the sum of the volumes of the individual components.
- **ΔH (change in enthalpy)** is also greater than 0, indicating that the mixing process is endothermic.

**Hint:** Remember that positive deviation is associated with weaker interactions between different molecules compared to interactions in pure substances.

### Step 4: Azeotropes
In the case of positive deviation, a minimum boiling azeotrope can form. An azeotrope is a mixture that has a constant boiling point and composition throughout the distillation process, meaning that the proportions of the components cannot be changed by simple distillation.

**Hint:** Consider the definition of azeotropes and how they relate to boiling points and vapor pressures.

### Step 5: Evaluating Statements
Now, we evaluate the statements given in the question:
1. The vapor pressure observed will be less than calculated from Raoult's law. (Incorrect)
2. ΔH mixture is less than zero. (Incorrect)
3. ΔV mixture is less than zero. (Incorrect)
4. The vapor pressure observed will be greater than calculated from Raoult's law. (Correct)

**Hint:** Compare each statement against the characteristics of positive deviation.

### Conclusion
The correct statement about a solution showing positive deviation is that the vapor pressure observed will be greater than calculated from Raoult's law. Therefore, the correct option is the fourth statement.