Solutions and solubilities

The ability of liquids to dissolve solids, other liquids, or gases has long been recognized as one of the fundamental phenomena of nature encountered in daily life. The practical importance of solutions and the need to understand their properties have challenged numerous writers since the Ionian philosophers and Aristotle. Though many physicists and chemists have devoted themselves to a study of solutions, as of the early 1990s it was still an incompletely understood subject under active investigation.

A solution is a mixture of two or more chemically distinct substances that is said to be homogeneous on the molecular scale—the composition at any one point in the mixture is the same as that at any other point. This is in contrast to a suspension (or slurry), in which small discontinuous particles are surrounded by a continuous fluid. Although the word solution is commonly applied to the liquid state of matter, solutions of solids and gases are also possible; brass, for example, is a solution of copper and zinc, and air is a solution primarily of oxygen and nitrogen with a few other gases present in relatively small amounts.

The ability of one substance to dissolve another depends always on the chemical nature of the substances, frequently on the temperature, and occasionally on the pressure. Water, for example, readily dissolves methyl alcohol but does not dissolve mercury; it barely dissolves benzene at room temperature but does so increasingly as the temperature rises. While the solubility in water of the gases present in air is extremely small at atmospheric pressure, it becomes appreciable at high pressures where, in many cases, the solubility of a gas is (approximately) proportional to its pressure. Thus, a diver breathes air (four-fifths nitrogen) at a pressure corresponding to the pressure around him, and, as he goes deeper, more air dissolves in his blood. If he ascends rapidly, the solubility of the gases decreases so that they leave his blood suddenly, forming bubbles in the blood vessels. This condition (known as the bends) is extremely painful and may cause death; it can be alleviated by breathing, instead of air, a mixture of helium and oxygen because the solubility of helium in blood is much lower than that of nitrogen.

The solubility of one fluid in another may be complete or partial; thus, at room temperature water and methyl alcohol mix in all proportions, but 100 grams of water dissolve only 0.07 gram of benzene. Though it is generally supposed that all gases are completely miscible—i.e., mutually soluble in all proportions—this is true only at normal pressures. At high pressures pairs of chemically unlike gases may exhibit only limited miscibility; for example, at 20° C helium and xenon are completely miscible at pressures below 200 atmospheres but become increasingly immiscible as the pressure rises.

The ability of a liquid to dissolve selectively forms the basis of common separation operations in chemical and related industries. A mixture of two gases, carbon dioxide and nitrogen, can be separated by bringing it into contact with ethanolamine, a liquid solvent that readily dissolves carbon dioxide but barely dissolves nitrogen. In this process, called absorption, the dissolved carbon dioxide is later recovered, and the solvent is made usable again by heating the carbon dioxide-rich solvent, since the solubility of a gas in a liquid usually (but not always) decreases with rising temperature. A similar absorption operation can remove a pollutant such as sulfur dioxide from smokestack gases in a plant using sulfur-containing coal or petroleum as fuel.

The process wherein a dissolved substance is transferred from one liquid to another is called extraction. As an example, phenolic pollutants (organic compounds of the types known as phenol, cresol, and resorcinol) are frequently found in industrial aqueous waste streams, and, since these phenolics are damaging to marine life, it is important to remove them before sending the waste stream back to a lake or river. One such removal technique is to bring the waste stream into contact with a water-insoluble solvent (e.g., an organic liquid such as a high-boiling hydrocarbon) that has a strong affinity for the phenolic pollutant. The solubility of the phenolic in the solvent divided by that in water is called the distribution coefficient, and it is clear that for an efficient extraction process it is desirable to have as large a distribution coefficient as possible.

Classes of solutions

Electrolytes and nonelectrolytes

Broadly speaking, liquid mixtures can be classified as either solutions of electrolytes or solutions of nonelectrolytes. Electrolytes are substances that can dissociate into electrically charged particles called ions, while nonelectrolytes consist of molecules that bear no net electric charge. Thus, when ordinary salt (sodium chloride, formula NaCl) is dissolved in water, it forms an electrolytic solution, dissociating into positive sodium ions (Na⁺) and negative chloride ions (Cl^-), whereas sugar dissolved in water maintains its molecular integrity and does not dissociate. Because of its omnipresence, water is the most common solvent for electrolytes; the ocean is a solution of electrolytes. Electrolyte solutions, however, are also formed by other solvents (such as ammonia and sulfur dioxide) that have a large dielectric constant (a measure of the ability of a fluid to decrease the forces of attraction and repulsion between charged particles). The energy required to separate an ion pair (i.e., one ion of positive charge and one ion of negative charge) varies inversely with the dielectric constant, and, therefore, appreciable dissociation into separate ions occurs only in solvents with large dielectric constants.

Most electrolytes (for example, salts) are nonvolatile, which means that they have essentially no tendency to enter the vapour phase. There are, however, some notable exceptions, such as hydrogen chloride (HCl), which is readily soluble in water, where it forms hydrogen ions (H⁺) and chloride ions (Cl^-). At normal temperature and pressure, pure hydrogen chloride is a gas, and, in the absence of water or some other ionizing solvent, hydrogen chloride exists in molecular, rather than ionic, form.

Solutions of electrolytes readily conduct electricity, whereas nonelectrolyte solutions do not. A dilute solution of hydrogen chloride in water is a good electrical conductor, but a dilute solution of hydrogen chloride in a hydrocarbon is a good insulator. Because of the large difference in dielectric constants, hydrogen chloride is ionized in water but not in hydrocarbons.