Solution
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In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent and solute particles are greater than the attractive forces holding the solute particles together, the solvent particles pull the solute particles apart and surround them. These surrounded solute particles then move away from the solid solute and out into the solution. The mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution usually has the state of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case. One important parameter of a solution is the concentration, which is a measure of the amount of solute in a given amount of solution or solvent.
Contents
Types
Gaseous mixtures
If the solvent is a gas, only gases (non-condensable) or vapors (condensable) are dissolved under a given set of conditions. An example of a gaseous solution is air (oxygen and other gases dissolved in nitrogen). Since interactions between gaseous molecules play almost no role, non-condensable gases form rather trivial solutions. In the literature, they are not even classified as solutions, but simply addressed as homogeneous mixtures of gases. The Brownian motion and the permanent molecular agitation of gas molecules guarantee the homogeneity of the gaseous systems.
Liquid solutions
If the solvent is a liquid, there are three subcategories:
- Gas in liquid: Oxygen, HCl in water; in some cases, like carbon dioxide, the gas reacts with water forming carbonic acid.
- Liquid in liquid: liquids can dissolve in other liquids either partially (ex: nitromethane partially dissolves in water) or completely (ethanol and water are mutually soluble in any proportion), the latter property is called miscibility.
- Solid in liquid: Ionic compounds, like sodium chloride dissolve in water, dissociates into ions. Other compounds, like glucose will dissolve in water without dissociating.
Solutions in water are especially common, and are called aqueous solutions. Non-aqueous solutions are when the liquid solvent involved is not water.
Solid solutions
Like above, can also be split into three sub categories:
- Gas in solids: Hydrogen dissolves rather well in metals, especially in palladium; this is studied as a means of hydrogen storage.
- Liquid in solid: Mercury in gold or sodium, forming an amalgam; polymers containing plasticizers such as phthalate (liquid) in PVC (solid).
- Solid in solid: Every single alloy in existence: steel, basically a solution of carbon atoms in a crystalline matrix of iron atoms; bronze, a solution of tin in copper. Glass and ceramic materials are sometimes classified as solid solutions of various metallic oxides in silicon dioxide or some other oxide.
Properties
The ability of one compound to dissolve in another compound is called solubility. When a liquid can completely dissolve in another liquid the two liquids are miscible. Two substances that can never mix to form a solution are said to be immiscible.
The physical properties of compounds such as melting point and boiling point change when other compounds are added. There are several ways to quantify the amount of one compound dissolved in the other compounds collectively called concentration. Examples include molarity, volume fraction, and mole fraction.
