Which of the following statement is incorrect regarding positive deviation from ideal behaviour

Options

A. Force of interaction decreases
B. Volume increases
C. Observed vapour pressure is more than expected vapour pressure
D. Gibbs energy change `(DeltaG)` is positive

Answer

To determine which statement is incorrect regarding positive deviation from ideal behavior, we need to understand the key characteristics of positive deviation in solutions.

### Step-by-Step Solution:

1. **Understanding Positive Deviation**:
- Positive deviation from ideal behavior occurs when the vapor pressure of a solution is higher than what is predicted by Raoult's Law. This means that the actual vapor pressure exceeds the expected vapor pressure based on the mole fractions of the components.

2. **Raoult's Law**:
- According to Raoult's Law, the vapor pressure of a component in a solution (P) is given by:
\[ P_A = P^0_A \cdot X_A \]
\[ P_B = P^0_B \cdot X_B \]
- Where \(P^0_A\) and \(P^0_B\) are the vapor pressures of the pure components A and B, and \(X_A\) and \(X_B\) are their respective mole fractions in the solution.

3. **Conditions for Positive Deviation**:
- In a solution exhibiting positive deviation:
- The observed vapor pressure (P) is greater than the expected vapor pressure (P_A or P_B calculated from Raoult's Law).
- The forces of attraction between solute and solvent molecules are weaker than the forces of attraction between like molecules (solute-solute and solvent-solvent).

4. **Characteristics of Positive Deviation**:
- The enthalpy of mixing (\(\Delta H_{mix}\)) is positive, indicating that the process absorbs heat.
- The volume of the mixture (\(\Delta V_{mix}\)) is also positive, indicating that the volume expands upon mixing.
- Gibbs free energy change (\(\Delta G\)) is not necessarily positive or negative in this context; it does not directly relate to the positive deviation.

5. **Evaluating the Statements**:
- **Statement 1**: "Forces of interaction decrease." - This is true.
- **Statement 2**: "Volume increases." - This is true.
- **Statement 3**: "Observed vapor pressure is more than the expected vapor pressure." - This is true.
- **Statement 4**: "Gibbs free energy change is positive." - This is incorrect; Gibbs free energy change does not have a defined sign in the context of positive deviation.

### Conclusion:
The incorrect statement regarding positive deviation from ideal behavior is **Statement 4**: "Gibbs free energy change is positive."