The key difference between positive and negative deviations in non-ideal solutions lies in how the observed vapor pressure of the components compares to the predictions based on Raoult’s law.

Positive Deviations:

• Positive deviations occur when the observed vapor pressure of a component in a non-ideal solution is higher than predicted by Raoult’s law.
• This deviation suggests that the intermolecular forces between the components in the mixture are weaker than the forces within the pure components.
• Positive deviations often result in enhanced vapor pressure and increased volatility of the components compared to what would be expected in an ideal solution.
• These mixtures tend to exhibit stronger interactions between different components, leading to increased molecular attractions and a tendency for the components to escape more easily into the vapor phase.

Negative Deviations:

• Negative deviations occur when the observed vapor pressure of a component in a non-ideal solution is lower than predicted by Raoult’s law.
• This deviation indicates that the intermolecular forces between the components in the mixture are stronger than the forces within the pure components.
• Negative deviations typically result in reduced vapor pressure and lower volatility of the components compared to an ideal solution.
• These mixtures often involve stronger interactions between the components, which can lead to increased molecular attractions and a tendency for the components to remain in the liquid phase rather than escaping into the vapor phase.

In summary, positive deviations in non-ideal solutions involve weaker intermolecular forces between the components, leading to higher observed vapor pressures and increased volatility. Negative deviations, on the other hand, arise from stronger intermolecular forces between the components, resulting in lower observed vapor pressures and reduced volatility. These deviations from Raoult’s law highlight the complex interactions and behaviors exhibited by non-ideal solutions.