In chemistry, a solution is a type of homogeneous mixture consisting of two or more substances in which one substance (the solute) is dissolved in another (the solvent). Because the mixture is homogeneous, a sample of a solution has the same appearance and concentration as any other sample. Although liquid and gaseous solutions are most familiar, a solution may exist in any phase. Usually, this is the phase of the solvent.
Components of a Solution
A solution consists of two main parts. The solvent is the component that is present in the greatest amount, while the solute (or solutes) occur in lesser quantities. A typical solution, like salt water, consists of a liquid solvent (water) and solid solute (sodium chloride). If the solvent is water, then the result is an aqueous solution. However, the solvent and solute can be any state of matter. For example, a solution of ethanol and water consists of two liquids. Which is the solvent and which is the solute depends on the one present in the greater concentration.
While the components of the solution do not necessary have the same phase, the solution consists of a single phase. Usually, this is the phase of the solvent. Also, the solute particles are evenly distributed (homogeneous) and too small to see. In contrast, a suspension is a heterogeneous mixture where the solute particles are often visible.
Examples of Solutions
The solute and solvent may be the same phase as one another (e.g., gases in air) or in different phases (e.g., dissolving salt in water to make saline solution). The end result is always a single phase. Here are some examples of solutions in different phases:
| Solution Type | Example |
|---|---|
| gas-gas | air (oxygen, carbon dioxide are solutes; nitrogen is the solvent) |
| gas-liquid | carbon dioxide in soda (carbon dioxide is the solute; water is the solvent) |
| gas-solid | hydrogen gas in palladium metal |
| liquid-liquid | gasoline |
| solid-liquid | sugar in water (sugar is the solute; water is the solvent) |
| liquid-solid | mercury dental amalgam |
| solid-solid | sterling silver (copper is the solute; silver is the solvent) |
Sometimes it’s difficult to identify the solute and the solvent in a solution. For example, if you mix equal parts of ethanol and water or equal volumes of nitrogen and oxygen, which is the solvent? If the mole fractions of the components are equal, it’s fine to assign the roles of solute and solvent. However, if one of the substances is water, it’s usually considered the solvent.
Properties of Solutions
There are several properties of chemical solutions that distinguish them from other types of mixtures:
- A solution is homogeneous.
- A solution consists of a single phase (e.g., liquid, solid, or gas).
- A solution does not scatter a beam of light.
- Particles in a solution are too small to be visible to the naked eye.
- Components of a solution cannot be separated using simple mechanical filtration.
Types of Solutions and Solubility
The amount of solute that will dissolve in a solvent is its solubility. Solubility depends on temperature and other factors. Usually, solubility increases with temperature. Chemical solutions may be classified according to the amount of solute they contain:
- Dilute solution: In a dilute solution, the amount of solvent is much greater than the amount of solute. Usually, mole fractions are used to describe the amount of solute and solvent.
- Concentrated solution: A concentrated solution contains the maximum amount of solute that will dissolve in a solvent or nearly the maximum amount.
- Saturated solution: A saturated solution is the type of concentrated solution that contains the maximum amount of dissolved solute, based on solubility.
- Supersaturated solution: A supersaturated solution contains more than the maximum amount of solute that will dissolve in the solvent. Usually, this type of solution is made by saturating a solution at a higher temperature and then carefully cooling it to a lower temperature.
References
- IUPAC (1997). “Solution.” Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). doi:10.1351/goldbook.S05746
- Lew, Kristi (2009). “Homogeneous.” Acids and Bases, Essential Chemistry. New York: Chelsea House Publishing. Online publisher: Science Online. Facts On File, Inc. ISBN 978-0-7910-9783-0.