Limiting Solution Behavior

It can be shown that the ideal solution represents the limiting behavior of very dilute solutions. The question may be posed: ``In what cases can we expect the activity to depend only on composition?''

Consider a very dilute solution of $B$ in $A$:

Figure 31-7: An atomic idealization of a dilute condensed solution of $B$ dissolved in $A$ in equilibrium with its vapor.

Each time an $A$ comes out of solution, it does so mostly without any influence of $B$. It is as if it does so as in a pure solution.

Each time a $B$ comes out of solution, it does so entirely under the influence of the surrounding $A$ atoms; it is as if it does so from pure $A$.

So one can expect very concentrated, or very dilute solutions to behave ideally.

Typically, the data look like the following:

Figure 31-8: Typical data for the behavior of solutions.

One gets as limiting behavior:

Raoult's Law:¹

$$\input{equations/raoult}$$ (31-5)

and Henry's law:

$$\input{equations/henry}$$ (31-6)