Introductory University Chemistry I
Solutions and Solubility
Dissolution, Evolution, and Precipitation
Pure substances, whether elements or compounds, are rarely found in nature and can often
be obtained only with considerable difficulty. Most materials we encounter are mixtures of two or
more substances. When a mixture is homogeneous, it is called a solution. A mixture of grains of sand
and grains of salt is not homogeneous, while a mixture of salt and water forms a homogeneous
mixture or solution when enough water is present to form a homogeneous mixture with, or to
dissolve, all of the salt present. Likewise, gases dissolve in liquids to form solutions. Fish can survive
in water by using their gills to extract the dissolved oxygen.
Much of the world around us is made up of aqueous solutions. The oceans and our blood are
only two of many examples. We now take up some of the aspects of solutions will be considered, with
emphasis on solutions in that almost ubiquitous solvent, water. The same concepts apply equally
well to solutions which are not aqueous.
Dissolution and Precipitation
The process of dissolution occurs when a solute is placed in contact with a solvent and
dissolves to form a solution. If solvent is removed from a solution, by a process such as evaporation,
eventually the solute will be found to separate from the remaining solution. This separation is called
the precipitation of a solid or the evolution of a gas. The concentration of solute which the solution
contains at the onset of precipitation or gas evolution is the maximum concentration of solute which
the solution can contain. This concentration is called the solubility of the solute. The solubility is often
expressed in units of mol/dm3 and is then called the molar solubility.
A solution which contains the maximum concentration of solute which it can contain under
the current conditions, which is the molar solubility, is called a saturated solution. A solution which
contains less than this maximum concentration is called an unsaturated solution. It is sometimes
possible to prepare solutions which are more concentrated than a saturated solution, although these
solutions are unstable and the excess solute will sooner or later separate from the solution. Such
solutions are called supersaturated solutions. A supersaturated solution will often lose the excess
solute immediately if a suitable site on which to do so, such as a scratched beaker wall, a gas bubble,
or a crystal of solute, is introduced into the solution.
Solubility is a Dynamic Equilibrium
The processes of dissolution and precipitation are the reverse of each other. Taken together
they form a dynamic equilibrium. Whenever a supersaturated solution forms, the equilibrium state
will sooner or later be achieved by precipitation of a solid salt. Whenever an unsaturated solution is
present in contact with a solid salt, the equilibrium state will sooner or later be achieved by dissolution
of all or part of the solid salt.
Dissolution of a solid salt does not take place uniformly from the surface of salt crystals.
Sections of the solid salt having a greater surface-to-volume ratio, such as small cryals or projections
from the surface, tend to dissolve more rapidly than the bulk of the crystals. Likewise, precipitation
does not occur uniformly throughout a solution. Precipitation proceeds through two stages called
nucleation and crystal growth. Nucleation is the process of formation of tiny crystalline nuclei in the
solution, while crystal growth is the ordered growth of these nuclei into larger well-formed crystals.
Crystal growth can occur only after nuclei are formed. Supersaturated solutions are solutions in which
significant nucleation has not yet taken place, which explains why the addition of a crystal which can
serve as a nucleus, or dust or a scratch which can serve as a nucleus as well, produces rapid crystal
formation in supersaturated solutions.
The condition of a precipitated salt left in contact with the saturated solution from which it
precipitated is observed to change. Over a period of minutes to hours or days the smaller crystals
dissolve while the larger crystals grow larger still. Chemists take advantage of this process of
conditioning or digestion of precipitates in order to obtain larger crystals which can be filtered from
the remaining solution, or supernatant, more easily.
Dissolution of a gas is likewise a surface process; it takes place at the gas-solution interface. Solutions
of gases will approach equilibrium much more rapidly when the solution is stirred to distribute the
dissolved gas throughout the solvent and when the area of the gas-solution interface is increased. In
tanks containing tropical fish, bubblers are used to maintain the equilibrium partial pressure of the
oxygen dissolved in the tank.
[NEXT: Solubility of Gases: Henry's Law]
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Copyright 1995 James A. Plambeck (Jim.Plambeck@ualberta.ca).
Updated November 4, 1995 jp.