Page 242 - Physical chemistry understanding our chemical world

PHASE EQUILIBRIA INVOLVING TWO-COMPONENT SYSTEMS: PARTITION 209

this last portion solidifies, the amount of oxygen in solution exceeds K (partition) and
leaves solution as gaseous oxygen. It is this expelled oxygen we see as tiny bubbles
of gas.

Aside

Zone refining is a technique for decreasing the level of impurities in some metals, alloys,
semiconductors, and other materials; this is particularly so for doped semiconductors, in
which the amount of an impurity must be known and carefully controlled. The technique
relies on the impurities being more soluble in a molten sample (like oxygen in water,
as noted above) than in the solid state.
To exploit this observation, a cylindrical bar of material is passed slowly through
an induction heater and a narrow molten ‘zone’ is moved along its length. This causes
the impurities to segregate at one end of the bar and super-pure material at the other.
In general, the impurities move in the same direction as the molten zone moves if the
impurities lower the melting point of the material (see p. 212).

How does recrystallization work?

Partition and the solubility product

We say the solution is saturated if solute is partitioned between a liquid-phase solution
and undissolved, solid material (Figure 5.16). In other words, the solution contains
as much solute as is feasible, thermodynamically, while the remainder remains as
solid. The best way to tell whether a solution is saturated, therefore, is to look for
undissolved solid. If K (partition) is small then we say that not much of the solute resides
in solution, so most of the salt remains as solid – we say the salt is not very soluble.
Conversely, most, if not all, of the salt enters solution if K (partition) is large.
Like all equilibrium constants, the value of K (partition) depends on temperature,
sometimes strongly so. It also depends on the solvent polarity. For example, K (partition)

Solution saturated
with solute

Solid crystals
of solute

Figure 5.16 In a saturated solution, the solute is partitioned between the solid state and solute
in solution