Physical Chemistry , 1st ed.

7.6 Liquid/Solid Solutions

In this section, we will consider only solutions in which the liquid component

has the majority mole fraction (the solvent) and the solid component has the

minority mole fraction (the solute). We will also assume that the solid solute is

non-ionic, because the presence of oppositely charged ions in solution affects

the properties of the solution (which will be considered in the next chapter).

There is also a consideration that is implicit in specifying a solid component:

it contributes nothing to the vapor phase that is in equilibrium with the solu-

tion. One way of speaking of this is to state that the solid is a nonvolatilecom-

ponent. Solutions of this sort are therefore easy to separate by simple distilla-

tionof the only volatile component, the solvent, rather than the more

complicated fractional distillation. Figure 7.20 shows two experimental setups

for simple distillation. Compare these to Figure 7.9.

Having mentioned the liquid-gas phase change for the liquid component,

what about the liquid-solid phase change? That is, what happens when the so-

lution is frozen? Typically, the freezing point of a solution is not the same as

the freezing point of the pure liquid, which is a topic discussed shortly. However,

when liquid solidifies,pure solid phase is formed.The remaining liquid phase

becomes more concentrated in solute, and this increase in concentration con-

tinues until the solution is saturated. Any further concentration causes precip-

itation of solute along with solidification of the solvent. This continues until

all of the solute is precipitated and all of the liquid component is pure solid.

Most liquid/solid solutions do not make solutions in infinite ratios.

Typically, there is a limit to how much solid can be dissolved in a given

amount of liquid. At this limit, the solution is said to besaturated.The solu-

bilityrepresents the amount of solid that is dissolved in order to make a sat-

urated solution, and is given in a wide variety of units. [A common unit for

7.6 Liquid/Solid Solutions 185

Sample

flask

Receiver

flask

Condenser

Thermometer

(a)

Figure 7.20 Apparatus for simple distillation. Compare these with Figure 7.9. (a) Normal-

scale simple distillation apparatus. (b) Microscale simple distillation apparatus.

Thermometer

Threaded cap

5.0 mL

conical vial

with magnetic

spin vane

(b)