Definitions

solution

solution

[suh-loo-shuhn]
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

The addition of some solutes to a solvent will raise the temperature of the solution, while others may lower the temperature and still others will have no noticeable effect. This behavior depends on the heat of solution of the solute in the given solvent. The heat of solution, i.e., the amount of heat given off or absorbed during the process of solution, is equal to the difference between the energy that must be supplied to break up the crystals of the solute and the energy that is released when the solute particles are taken into solution by the solvent (see enthalpy). If the heat of solution is negative (i.e., more energy is required to break up the crystal than is released in forming the solution), then the temperature will decrease; if the heat of solution is positive, the temperature will increase.

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. A common example is a solid, such as salt or sugar, dissolved in water, a liquid. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can combine with other gases to form mixtures, rather than solutions. All solutions are characterized by interactions between the solvent phase and solute molecules or ions that result in a net decrease in free energy. Under such a definition, gases typically cannot function as solvents, since in the gas phase interactions between molecules are minimal due to the large distances between the molecules. This lack of interaction is the reason gases can expand freely and the presence of these interactions is the reason liquids do not expand.

Examples of solid solutions are alloys, certain minerals and polymers containing plasticizers. The ability of one compound to dissolve in another compound is called solubility. The physical properties of compounds such as melting point and boiling point change when other compounds are added. Together they are called colligative properties. There are several ways to quantify the amount of one compound dissolved in the other compounds collectively called concentration. Examples include molarity, molality, and parts per million (ppm).

Solutions should be distinguished from non-homogeneous mixtures such as colloids and suspensions.

Types of solutions

Many types of solutions exist, as solids, liquids and gases can be both solvent and solute, in any combination:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Examples of solutions Solute
Gas Liquid Solid
Solvent Gas Oxygen and other gases in nitrogen (air) Water vapor in air Naphthalene slowly sublimes in air, going into solution.
Liquid Carbon dioxide in water (carbonated water; the visible bubbles, however, are not the dissolved gas, but only an effervescence; the dissolved gas itself is not visible in the solution) Ethanol (common alcohol) in water; various hydrocarbons in each other (petroleum) Sucrose (table sugar) in water; sodium chloride (table salt) in water; gold in mercury, forming an amalgam
Solid Hydrogen dissolves rather well in metals; platinum has been studied as a storage medium. Hexane in paraffin wax, mercury in gold. Steel, duralumin, other metal alloys

Solvation

During solvation, especially when the solvent is polar, a structure forms around it, which allows the solute-solvent interaction to remain stable.

When no more of a solute can be dissolved into a solvent, the solution is said to be saturated. However, the point at which a solution can become saturated can change significantly with different environmental factors, such as temperature, pressure, and contamination. For some solute-solvent combinations a supersaturated solution can be prepared by raising the solubility (for example by increasing the temperature) to dissolve more solute, and then lowering it (for example by cooling).

Usually, the greater the temperature of the solvent, the more of a given solid solute it can dissolve. However, most gases and some compounds exhibit solubility that decrease with increased temperature. Such behavior is a result of an exothermic enthalpy of solution. Some surfactants exhibit this behaviour. The solubility of liquids in liquids is generally less temperature-sensitive than that of solids or gases.

Ideal solutions

Properties of an ideal solution can be calculated by the linear combination of the properties of its components.

If both solute and solvent exist in equal quantities (such as in a 50% ethanol, 50% water solution), the concepts of "solute" and "solvent" become less relevant, but the substance that is more often used as a solvent is normally designated as the solvent (in this example, water).

See also

References

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