Which statement correctly describes when a solution deviates positively from Raoult's law?

• A. Negative deviation indicates stronger solute-solvent interactions.
• B. Positive deviation indicates weaker solute-solvent interactions.
• C. Raoult's law deviations indicate ideal behavior.
• D. Deviations have no relation to molecular interactions.

Answer: (B)

The key idea is how intermolecular forces between solute and solvent affect vapor pressure relative to Raoult’s law. Raoult’s law describes an ideal solution where each component contributes to the total vapor pressure proportional to its mole fraction and its pure-component vapor pressure. Deviations from this law reveal how real interactions differ from those in the pure liquids.

A positive deviation means the total vapor pressure is higher than the ideal prediction. This happens when the interactions between solute and solvent are weaker than the interactions present in the pure components. With weaker attraction between different molecules, they escape into the vapor more readily, driving the vapor pressure up above the ideal value.

So the statement that best captures this is that a positive deviation indicates weaker solute–solvent interactions. Conversely, negative deviation would reflect stronger solute–solvent interactions, which lower the vapor pressure below the ideal value. It’s also important to recognize that deviations signal non-ideal behavior and arise precisely because of differences in molecular interactions, not because there’s no relation to interactions.