Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
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|
|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|
Liquid solvents can be broadly classified into polar and non-polar solvents. A common measure of the polarity of a solvent is the dielectric constant. The most widely used polar solvent is water, with a dielectric constant of 78.5. Ethanol, with a dielectric constant of 24.3, has intermediate polarity. An example of a non-polar solvent is hexane, which has a dielectric constant of 1.9. Generally polar or ionic compounds will only dissolve in polar solvents. A simple test for the polarity of a liquid solvent is to rub a plastic rod, to induce static electricity. Then hold this charged rod close to a running stream of the solvent. If the path of the solvent deviates when the rod is held close to it, it is a polar solvent. Certain molecules have polar and non-polar regions, for example sodium dodecyl sulfate. This class of molecules (called amphipathic molecules) includes surfactants like soaps and emulsifiers, as they have the ability to stabilize emulsions by aligning themselves on the interface between the non-polar and polar liquids, with their polar ends in the polar liquid and their non-polar ends in the non-polar liquid.
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 deacrease 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.
The 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).
- Molar solution
- Percentage solution
- Solubility equilibrium
- Stock solution
- Total dissolved solids is a common term in a range of disciplines, and can have different meanings depending on the analytical method used. In water quality, it refers to the amount of residue remaining after evaporation of water from a sample.
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