Solution
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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 and gases always mix with other gases.[1]
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.
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[edit] 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) | ||
| 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. | Hexane in paraffin wax, mercury in gold. | Steel, duralumin, other metal alloys | |
[edit] Solvents
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.
[edit] 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 changes significantly with different environmental factors, such as temperature, pressure, and contamination. Raising the solubility (for example 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 solid 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. The solubility of liquids in liquids is generally less temperature-sensitive than that of solids or gases, while gases usually become less soluble with increasing temperature.
[edit] Ideal solutions
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).