What is indicated by a negative deviation from Raoult's Law concerning vapor pressure?

A negative deviation from Raoult's Law indicates that the vapor pressure of a solution is lower than what would be predicted based solely on the vapor pressures of its pure components. This situation arises due to the presence of stronger interactions between the molecules of different components compared to those between the molecules of the individual pure substances.

When a solution shows a negative deviation, it suggests that strong intermolecular forces such as hydrogen bonding, dipole-dipole interactions, or ionic interactions are at play between the mixed components. These stronger attractions require more energy to overcome when transitioning from liquid to vapor, resulting in a lower vapor pressure.

This understanding is crucial in the context of solution behavior. It contrasts with a positive deviation, where weaker interactions occur, leading to higher vapor pressure than predicted. Hence, the identification of stronger bonds in a solution is the key reason why the vapor pressure decreases in this scenario.