James Richard Fromm
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.
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/liter 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.
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 crystals 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.