Animated Solution for Chemistry - Solutions: In a mixture of $A$ and $B$, components show negative deviation when

Reality

Imagine you are a chemist mixing two volatile liquids, let's call them Liquid $A$ and Liquid $B$. In a perfectly ideal world, these two liquids would mix without any drama. They would follow Raoult's Law to the letter, meaning the total vapour pressure of the mixture would be exactly the sum of their individual partial pressures, calculated as $p_{\text{total}}=p_A^{\circ }{x}_A+p_B^{\circ }{x}_B$.

However, reality is rarely ideal. Often, when you mix two liquids, the resulting vapour pressure is either higher or lower than what Raoult's Law predicts. When the observed vapour pressure is lower than the expected ideal value, we say the solution exhibits a negative deviation from Raoult's Law.

To understand why this happens, we need to think about what vapour pressure actually represents. Vapour pressure is a measure of the "escaping tendency" of molecules from the liquid phase into the gas phase. If a liquid has a high vapour pressure, its molecules are escaping easily. If it has a low vapour pressure, the molecules are struggling to break free.

In a solution showing negative deviation, the total vapour pressure has dropped. This means that, overall, the molecules of $A$ and $B$ are finding it much harder to escape into the vapour phase than they did when they were pure liquids.

Why would molecules suddenly find it harder to escape? The answer lies in the microscopic tug-of-war of intermolecular forces.

Before mixing, the molecules in pure Liquid $A$ are held together by $A-A$ interactions, and those in pure Liquid $B$ are held together by $B-B$ interactions. When we mix them, the molecules are forced to mingle, creating new $A-B$ interactions.

If the new $A-B$ interactions are stronger than the original $A-A$ and $B-B$ interactions, the molecules of $A$ and $B$ will hold onto each other very tightly. Because they are so strongly attracted to one another, they require more energy to break these bonds and escape into the gas phase.

This strong mutual attraction drastically reduces their escaping tendency, which directly results in a lower vapour pressure for the entire solution.

Therefore, the fundamental cause of a negative deviation from Raoult's Law is that the $A-B$ interaction is stronger than the $A-A$ and $B-B$ interactions. This perfectly aligns with our understanding of molecular dynamics and leads us straight to the correct answer.