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solution, in chemistry, homogeneous mixture of two or more substances. The dissolving medium is called the solvent, and the dissolved material is called the solute. A solution is distinct from a colloid or a suspension.
In most common solutions, the solvent is a liquid, often water, and the solute may be a solid, gas, or liquid. For example, syrups are solutions of sugar, a solid, in water, a liquid; household ammonia is a solution of ammonia gas in water; and vinegar is a solution of acetic acid, a liquid, in water. When two liquids, e.g., water and ethanol, can be mixed in any proportions, the solvent is commonly considered to be the one present in greater proportion. Some alloys are solutions of one solid in another, as are many rocks. A mixture of gases, such as air, is usually not thought of as a solution.
Characteristics of Solutions
The solute particles in a solution are generally of molecular size or smaller, much smaller than those in a colloid or a suspension. The solute particles cannot be observed even with an ultramicroscope. They do not settle out from the solvent on standing, and they cannot be separated from the solvent by physical means, such as filtration or centrifugation. On the other hand, a solution differs from a compound in that its components can occur in continuously varying proportions, within certain limits (although within a given solution they are present in the same proportions throughout the solution), while the components of a compound can occur only in certain fixed proportions.
The addition of solute affects the boiling point, freezing point, and vapor pressure of the solution, in general raising the boiling point, depressing the freezing point, and lowering the vapor pressure (see Raoult's law). A number of substances (acids, bases, and salts) exhibit characteristic behavior in aqueous solution. These substances dissociate in water to form positive and negative ions that enable the solution to conduct electricity. Such solutions are called electrolytic (see electrolyte).
The proportion of solute to solvent in a given solution is expressed by the concentration of the solution. Concentrations may be stated in a number of ways, such as giving the amount of solute contained in a given volume of solution or the amount dissolved in a given mass of solvent. A solution having a relatively high concentration is said to be concentrated, and a solution having a low concentration is said to be dilute.
In many solutions the concentration has a maximum limit that depends on various factors, such as temperature, pressure, and the nature of the solvent. The maximum concentration is called the solubility of the solute under those conditions. When a solution contains the maximum amount of solute, it is said to be saturated; if it contains less than that amount, it is unsaturated.
The most obvious factor affecting solubility is the nature of the solvent. Ordinary table salt (sodium chloride) is soluble in water, but only slightly soluble in ethanol, and insoluble in diethyl ether. Temperature is also important in determining solubility. Solids are usually more soluble at higher temperatures; more salt will dissolve in warm water than in an equal amount of cold water. Graphs showing the solubility of different solids as a function of temperature are called solubility curves and are very useful in chemical analysis. Solubility also depends on pressure, especially in the case of gases, which are more soluble at higher pressures.
Under certain conditions a solution may be made to contain more solute than a saturated solution at the same temperature and pressure; such a solution is called supersaturated. If even a single crystal of undissolved solute is added to a supersaturated solution, all the excess solute above the normal solubility concentration will immediately crystallize out of the solution.
Heat of Solution
a macroscopically homogeneous mixture of two or more substances, or components, that form systems in thermodynamic equilibrium. All the components of a solution exist in a molecularly dispersed state and are uniformly distributed as separate atoms, molecules, or ions or as groups of a relatively small number of the above-mentioned particles. From a thermodynamic viewpoint, solutions are phases of variable composition in which the ratio of the components may vary continuously within certain limits under given external conditions. Solutions may be gaseous or solid, but term “solution” most frequently refers to a liquid solution.
Practically all liquids encountered in nature are solutions. For example, seawater is a solution of a large number of inorganic and organic substances in water, and petroleum is a solution of many components, which are usually organic. Solutions are found widely in industry and daily life.
The simplest components of solutions usually may be separated in pure form, and a solution of permissible composition can be formed by remixing such components. The quantitative ratio of components is determined by their concentrations. The major component is usually called the solvent, and the other components are called solutes. If one of the components is a liquid and the others are gases or solids, the liquid is considered to be the solvent.
The classification of solutions is based on various criteria. Thus, a distinction based on the concentration of the solute is made between concentrated and dilute solutions. The nature of the solvent determines whether a solution is aqueous or nonaqueous (alcoholic and ammonia solutions). Based on the concentration of hydrogen ions, a distinction is made between acid, neutral, and basic solutions.
In accordance with their thermodynamic properties, solutions are divided into certain classes, primarily into ideal and nonideal solutions. In ideal solutions the chemical potential μi, of each component i has a simple logarithmic dependence on its concentration (for example, on the mole fraction xi):
(1) where is the chemical potential of the pure component, which is dependent only on the pressure P and the temperature T, and where R is the gas constant.
For ideal solutions, the enthalpy of mixing of the components is equal to zero, and the entropy of mixing is given by the same formula as for ideal gases. The change in volume upon mixing the components is equal to zero. These three properties of an ideal solution fully characterize it and may be considered as criteria for ideal solutions. Both Raoult’s laws and Henry’s law are satisfied for ideal solutions. It has been found that solutions are ideal only if their components are similar to each other, primarily in regard to geometric configuration and molecular size. Solutions most similar to ideal solutions are mixtures of identical compounds containing different isotopes of the same element.
As a rule, equation (1) holds for ideal solutions in areas of concentration change. The concentrations at which marked deviations from the ideal are first observed in a given solution depend very strongly on the nature of the solution components. Most sufficiently dilute solutions behave as ideal solutions.
Solutions not having the properties of ideal solutions are called nonideal. For these solutions, the relation that holds is analogous to equation (1), but the concentration is replaced by the activity ai = γixi where ai is the activity of component i and γi, is the activity coefficient, which depends both on the concentration of the given component and of the other components and on the pressure and temperature.
Regular solutions form a large class of nonideal solutions. They are characterized by the same entropy of mixing as ideal solutions, but their enthalpy of mixing is nonzero and proportional to the logarithms of the activity coefficients. Athermic solutions form a special class in which the heat of mixing is equal to zero; the activity coefficients are determined only by the entropy term and are independent of the temperature. The theory of such solutions frequently permits the prediction of the properties of nonideal solutions, for example, in the case of nonpolar components with very different molecular volumes. Many solutions of high molecular weight compounds in ordinary solvents are similar to athermic solutions.
At a given temperature and pressure, the dissolution of one component in another usually involves, within some limits, a change in concentration. Solutions in equilibrium with one of the pure components are called saturated solutions, and the concentrations of such solutions are the solubility of the component. The dependence of the solubility on temperature and pressure is represented graphically by a solubility diagram. At concentrations below the solubility of the solute, a solution is unsaturated. If a solution does not contain crystallization nuclei, it may be cooled so that the concentration of the solute becomes greater than the solubility and the solution becomes supersaturated. A series of solution properties of practical importance is related to the change in the pressure of the saturated vapor of the solvent above a solution upon changing the concentration of the solute. Such properties include a lowering of the freezing point and an increase in the boiling point.
The structure of a solution is primarily determined by its components. If the components are similar in chemical structure and molecular size, the structure of the solution, in principle, will not differ from the structure of the pure liquids. Compounds that markedly differ in molecular structure and properties usually interact strongly with each other, leading to the formation of complexes in the solution that cause deviation from the ideal. The energy of formation of these complexes may reach several kJ/mole, indicating the presence of weak chemical interactions in the solution and the formation of some chemical compounds as new components of the solution. The interaction with solvent molecules for many compounds (for example, electrolytes) is accompanied by the opposite phenomenon, namely, dissociation. Upon dissolution in water, salts, acids, and bases partially or completely dissociate into ions, resulting in an increase in the number of different particles in the solution. Dissociation also characterizes other polar solvents. In electrolytic dissociation, the overall electrical neutrality is retained, and a layer of closely related solvent molecules (the solvation shell) forms around each ion. The structure of the solvent is retained in solutions at very low concentrations of the solute. As the concentration of the solute increases, new structures arise. For example, various crystalline hydrate structures arise in aqueous solutions. Large ions destroy the structure of the solvent, resulting in experimentally observed structural nonuniformities. Solutions of macromolecular compounds are characterized by specific features.
The statistical molecular theory of solutions has been developed only for the simplest classes of solutions. Thus, in considering the solutions of nonassociated liquids, the concept of solutions as statistical sets of solids (“spheres,” “ellipsoids,” and “rods”) that interact with each other according to a defined model law is used. For highly dilute solutions of electrolytes, the consideration is limited to only the electrostatic interaction of the ions as point charges or as spheres of a given radius.
REFERENCESKirillin, V. A., and A. E. Sheindlin. Termodinamika rastvorov. Moscow, 1956.
Shakhparonov, M. I. Vvedenle v molekuliarnuiu teoriiu rastvorov. Moscow, 1956.
Prigogine, I. The Molecular Theory of Solutions. Amsterdam, 1957.
Robinson, R., and R. Stokes. Rastvory elektrolitov. Moscow, 1963. (Translated from English).
Tager, A. A. Fiziko-khimiiapolimerov, 2nd ed. Moscow, 1968.
Kurs fizicheskoi khimii, 2nd ed., vols. 1–2. Edited by Ia. I. Gerasimov. Moscow, 1969–73.
N. F. STEPANOV
"Flash is a perfect image-streaming solution." "What is it?" "Um... about a thousand dollars."
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