The Foundations of Chemistry

frequently. Some of these collisions result in recrystallization. The rates of the two
opposing processes become equal after some time. The solid and dissolved ions are then
in equilibrium with each other.

Such a solution is said to be saturated.Saturation occurs at very low concentrations of
dissolved species for slightly soluble substances and at high concentrations for very soluble
substances. When imperfect crystals are placed in saturated solutions of their ions, surface
defects on the crystals are slowly “patched” with no net increase in mass of the solid.
Often, after some time has passed, we see fewer but larger crystals. These observations
provide evidence of the dynamic nature of the solubility equilibrium. After equilibrium is
established, no more solid dissolves without the simultaneous crystallization of an equal
mass of dissolved ions.
The solubilities of many solids increase at higher temperatures. Supersaturated solu-
tionscontain higher-than-saturated concentrations of solute. They can sometimes be
prepared by saturating a solution at a high temperature. The saturated solution is cooled
slowly, without agitation, to a temperature at which the solute is less soluble. At this point,
the resulting supersaturated solution is metastable(temporarily stable). This may be thought
of as a state of pseudoequilibrium in which the system is at a higher energy than in its
most stable state. In such a case, the solute has not yet become sufficiently organized for
crystallization to begin. A supersaturated solution produces crystals rapidly if it is slightly
disturbed or if it is “seeded” with a dust particle or a tiny crystal. Under such conditions
enough solid crystallizes to leave a saturated solution (Figure 14-5).

solid dissolved particles

dissolution

crystallization

Dynamic equilibria occur in all

saturated solutions; for instance, there

is a continuous exchange of oxygen

molecules across the surface of water

in an open container. This is fortunate

for fish, which “breathe” dissolved

oxygen.

14-5 Rates of Dissolution and Saturation 551

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 14.2, Solubility.

A tiny crystal of sodium acetate,

NaCH 3 COO, was added to a clear,

colorless, supersaturated solution of

NaCH 3 COO. This photo shows

solid NaCH 3 COO just beginning to

crystallize in a very rapid process.

Figure 14-5 Another method of seeding a supersaturated solution is by pouring it very
slowly onto a seed crystal. A supersaturated sodium acetate solution was used in these
photographs.