# Why do ideal solutions behave differently from non-ideal solutions?

May 12, 2017

You are asking why black bears are not brown bears,,,,,,

#### Explanation:

Ideal solutions obey Raoult's Law:

${P}_{\text{solution}} = {\chi}_{a} {P}_{A} + {\chi}_{b} {P}_{B}$, where ${\chi}_{i} {P}_{i}$ is the product of the mole fraction of component $i$ and ${P}_{i}$ is the vapour pressure.

Azeotropes are constant boiling mixtures that distil without change in composition. Azeotropes can show positive variation from $\text{Raoult's Law}$ (vapour pressure is greater than the sum of the weighted product of the mole fractions), or negative deviation (vapour pressure is less than the sum of the weighted product of the mole fractions).

And negative variation shows greater (attractive) interaction between different solvent molecules, and positive deviation shows greater repulsive interaction. Non-ideality is indicated when the solution vapour pressure departs from linearity as shown on the graph.

So the answer to why black bears are not brown bears is that they are different categories.