Positive Deviation

Positive deviation refers to the situation in which the observed properties of a mixture are higher than the values predicted by Raoult's law. Raoult's law states that the partial vapor pressure of each component in an ideal mixture is directly proportional to its mole fraction in the mixture.

In a mixture of two components, A and B, positive deviation occurs when the AB interaction is weaker than the AA and BB interactions.

Explanation

To understand why positive deviation occurs in this scenario, let's consider the molecular interactions at play in the mixture.

AB Interaction

When the AB interaction is weaker than the AA and BB interactions, it means that the attractive forces between A and B molecules are not as strong as the attractive forces between like molecules (AA and BB). This can be due to differences in polarity, size, or intermolecular forces between A and B.

AA and BB Interactions

On the other hand, the AA and BB interactions are stronger because the attractive forces between like molecules are maximized. This could be due to similar molecular structures, similar polarities, or similar intermolecular forces between A and A molecules and B and B molecules.

Molecular Interactions and Deviation

The strength of molecular interactions plays a crucial role in determining the behavior of a mixture. When the AB interaction is weaker than the AA and BB interactions, it leads to positive deviation from Raoult's law.

Explanation of Other Options

Let's briefly discuss the other options and why they are not correct:

a) AB interaction is stronger than AA and BB interaction: In this case, the attractive forces between A and B molecules would be stronger than the attractive forces between like molecules. This would result in negative deviation instead of positive deviation.

c) V mix = 0, S mix = 0: These conditions imply that there is no change in volume or entropy when the two components are mixed. However, this information does not provide any insight into the strength of the molecular interactions, so it cannot explain positive deviation.

d) V mix = 0, S mix > 0: These conditions suggest that there is no change in volume but an increase in entropy when the components are mixed. Again, this information does not provide any information about the strength of the molecular interactions, so it cannot explain positive deviation.

Therefore, the correct answer is option 'B' where positive deviation occurs when the AB interaction is weaker than the AA and BB interactions.