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In chemistry, a solution is a homogeneous mixture composed of one or more substances, known as solutes, 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 and gases in other gases.
An ideal solution is one where the interactions of the molecules of the solvent with each other are equal to their interactions with the solutes. The properties of an ideal solution can be calculated by the linear combination of the properties of its components.
The solvent is conventionally defined as the substance that exists in a greater quantity than the solute(s) in the solution. 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).
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 dielectic constant of 24.3, has intermediate polarity. An example of a non-polar solvent is hexane, which has a dielectic 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 changes significantly with different environmental factors, such as temperature, pressure, and contamination. Raising the solubility (such as by increasing the temperature) to dissolve more solute, and then lowering the solubility causes a solution to become supersaturated.
In general the greater the temperature of a solvent, the more of a given solute it can dissolve. However, some compounds exhibit reverse solubility, which means that as a solvent gets warmer, less solute can be dissolved. Some surfactants exhibit this behaviour.
There are several ways to measure the strength of a solution; see concentration for more information.
There are many types of solutions:
|Examples of solutions||Solute|
|Solvent||Gas||Oxygen and other gases in nitrogen (air)||Water vapor in air (humidity)||The odor of a solid results from molecules of that solid being dissolved in the air|
|Liquid||Carbon dioxide in water (carbonated water)||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||Water in activated charcoal; moisture in wood||Steel, duralumin, other metal alloys|
- Colligative properties
- Molar solution
- Percentage solution
- Solubility equilibrium
- Suspension (chemistry)