Solutions and Their Concentrations

The majority of chemical processes are reactions that occur in solution. Important industrial processes often utilize solution chemistry. Air, tap water, tincture of iodine, beverages, and household ammonia are common examples of solutions. A solution is a homogenous mixture of substances with variable composition. The substance present in the major proportion is called the solvent, whereas the substance present in the minor proportion is called the solute. It is possible to have solutions composed of several solutes. The process of a solute dissolving in a solute is called dissolution.

Many common mixtures are heterogeneous, the components and properties of such mixtures are not distributed uniformly throughout their structures. Conversely, solutions are said to be homogeneous because they have uniform composition and properties. Solutions are intimate and random homogeneous mixtures of atomic-size chemical species, ions, or molecules. In addition to their observed homogeneity, true solutions also have certain other characteristics. For example, components of a solution never separate spontaneously, even when a significant density difference exists between the components. Solutions also pass through the finest filters unchanged.

The components of a solution distribute themselves in a completely random manner, given sufficient time. For example, a lump of sugar dropped into a glass of water dissolves, and eventually molecules of sugar can be found randomly distributed throughout the water, even though no mechanical stirring has been employed. This phenomenon, called diffusion, is similar to the process of diffusion that occurs with gases. The molecules of sugar as well as those of water must be in constant motion in the solution. In the case of liquid solutions, the sugar molecules do not move very far before they encounter other molecules; diffusion in a liquid is therefore less rapid than diffusion in a gas.

Many commonly encountered solutions are those involving a solid that has been dissolved in a liquid, but there are as many types of solutions as there are different combinations of solids, liquids, and gases. Solutions in which the solvent is a liquid and the solute is a gas, liquid, or solid are very common. The atmosphere is a good example of a solution in which a gaseous solvent (nitrogen) dissolves other gases (such as oxygen, carbon dioxide, water vapor, and neon). Solutions of solids in solids are another example, and these are encountered most often among the various metal alloys. Of all the liquid solvents used in the laboratory, in industry, and in the home, water is the most commonly employed and is the best of the inorganic solvents. The alcohols and numerous other types of compounds are classified as organic solvents; many of these are used in dry cleaning chemicals, nail polish removers, paint thinners, and many other similar purposes.

The concentration of a solution is defined as the amount of solute present in a given quantity of solvent. Very often scientists speak of concentrated solutions, dilute solutions, or very dilute solutions, but these designations give only a rough relative qualitative idea of concentration. For example, a "concentrated solution" contains a considerable quantity of solute as compared with a "dilute solution." Although such designations are only qualitatively useful, they are nevertheless widely used. The most common way to express concentration is on the basis of the weight of solute per unit weight of solvent. For example, a salt solution may be prepared by dissolving 1.64 grams of sodium chloride in 100 grams of water. The concentration of this solution could also be expressed as 0.0164 grams of NaCl per 1 gram of water, or as 16.4 grams of NaCl per 1,000 grams of water. Thus, a statement of the concentration of a solution does not imply anything concerning the amount of solute or the amount of solvent present, but rather gives the ratio of solute to solvent in terms of some convenient (and arbitrary) units. Because the weight of a sample of a liquid is usually more difficult to determine experimentally than its volume, a practical unit of concentration is the weight of solute in a given volume of the solution; for example, a sugar solution may contain 50 grams of sugar per 100 milliliters of solvent. The composition of many solutions cannot be varied continuously because there are certain fixed limits imposed by the nature of the substances involved. Solid salt and sugar can be mixed in any desired proportions, but unlimited quantities of sugar (or salt) cannot be dissolved in a given amount of water; however, up to the solubility limit, solutions can be produced in any desired proportion.

When the solvent contains a maximum quantity of solute, the resulting solution is said to be saturated. The saturation point varies according to the solute. For example, 100 grams of pure water at 25°C (77°F) can dissolve no more than 35.92 grams of NaCl to form a stable saturated solution, but this same amount of water at 25°C dissolves only 0.0013 grams of calcium carbonate. The solubility in these examples is expressed in grams of solute per 100 grams of water, but any suitable units could be used. Water can dissolve any amount of a solute less than that required for a saturated solution. Tables of the solubilities of many substances can be found in various chemistry texts.

In some cases there is no upper limit to the amount of a solute that a given quantity of solvent can dissolve, and these substances are said to be miscible in all proportions. Completely miscible substances give homogeneous mixtures (solutions); for example, a mixture of any two gaseous substances is homogeneous. Often, liquids such as alcohol and water can be mixed in all proportions to give homogeneous mixtures. When a saturated solution has been achieved, a dynamic equilibrium exists between the solute in solution and any undissolved solute. Molecules of the solute (or atoms or ions, depending upon the nature of the solute) are continuously going into solution, but since the solution is already saturated, an equal number of molecules of the solute leave the solution and redeposit on the excess solid solute. A state of equilibrium exists when these two processes occur at the same rate, the net result being a constant amount of solute in solution. A saturated solution can therefore be defined more precisely as a solution that is in equilibrium with an excess of the solute at a given temperature.

In some instances it is possible to prepare a true solution that contains an excess of the equilibrium amount of solute; this condition is called super saturation. Supersaturated solutions are unstable. If left undisturbed, they may remain in this state for an indefinite period of time. However, the excess solute can be brought out of solution by a slight agitation or by the addition of any solid particle (dust, a small crystal of solute, etc.) that can act as a center for crystal growth, returning the solution to its normal saturated state.