# What is the difference between ideal and nonideal binary solutions? What are examples?

Jan 6, 2017

Ideal solutions assume all intermolecular forces between mixed components are identical ($2 {\epsilon}_{A B} = {\epsilon}_{A A} + {\epsilon}_{B B}$). As a result, the average distance between particles does not change after mixing.

(These solutions do exist. Benzene and toluene form an essentially ideal solution, for example.)

Nonideal solutions account for the fact that sometimes, the intermolecular forces acquired are different after mixing than before mixing. As such, there are two common variations:

• $2 {\epsilon}_{A B} < {\epsilon}_{A A} + {\epsilon}_{B B}$
The intermolecular forces are more attractive after mixing.

This is called negative deviation, because the particles are more attracted to each other, meaning that the average distance is closer. Therefore, the solution contracts after mixing.

EXAMPLE: ${\text{HNO}}_{3}$ and $\text{H"_2"O}$.

The nitric acid dissociates in water, forming ${\text{H"_3"O}}^{+}$ and ${\text{NO}}_{3}^{-}$. The resultant ionic interactions pull the ions together and contract the solution volume.

• $2 {\epsilon}_{A B} > {\epsilon}_{A A} + {\epsilon}_{B B}$
The intermolecular forces are less attractive after mixing.

This is called positive deviation, because the particles are less attracted to each other, meaning that the average distance is farther. Therefore, the solution expands after mixing.

EXAMPLE: $\text{CH"_3"CH"_2"OH}$ and $\text{H"_2"O}$.

When these mix, the hydrophobic end of ethanol hates water, so water molecules tend to favor interacting with the $\text{OH}$ end (the hydrophilic, hydrogen-bonding end). This puts more distance between the two components, expanding the solution volume.