Which one is not equal to zero for an ideal solution?

Options

A. `DeltaP=P_("observed")-P_(Raoult)`
B. `DeltaH_(mix)`
C. `DeltaS_(mix)`
D. `DeltaV_(mix)`

Explanation

To determine which quantity is not equal to zero for an ideal solution, we need to analyze the properties of ideal solutions based on Raoult's law.

### Step-by-Step Solution:

1. **Understanding Ideal Solutions**: An ideal solution is one that follows Raoult's law, which states that the partial vapor pressure of each component in the solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution.

2. **Forces of Attraction**: In an ideal solution, the intermolecular forces between the molecules of the different components (A and B) are similar to the forces present in the pure components. This means that there is no significant change in the nature of the interactions when the two components are mixed.

3. **Change in Enthalpy (ΔH)**: Since the intermolecular forces do not change upon mixing, there is no energy absorbed or released during the mixing process. Therefore, the change in enthalpy (ΔH) for an ideal solution is zero.

4. **Change in Volume (ΔV)**: Similarly, because the interactions remain the same, there is no change in volume upon mixing. Thus, the change in volume (ΔV) is also zero.

5. **Change in Vapor Pressure (ΔP)**: The vapor pressure of the solution remains the same as that of the pure components because the nature of the interactions does not change. Therefore, the change in vapor pressure (ΔP) is zero.

6. **Change in Entropy (ΔS)**: When two different substances are mixed, the number of possible arrangements of the molecules increases, leading to an increase in randomness or disorder in the system. This results in a positive change in entropy (ΔS mix), which is greater than zero.

### Conclusion:
The quantity that is not equal to zero for an ideal solution is the change in entropy (ΔS mix).

**The correct answer is: ΔS mix > 0.**