Editing Solution (chemistry)

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{{short description|Homogeneous mixture of a solute and a solvent}}
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[[Image:SaltInWaterSolutionLiquid.jpg|thumb|upright|Making a [[saline water]] solution by dissolving [[Salt|table salt]] ([[sodium chloride|NaCl]]) in [[water]]. The salt is the solute and the water the solvent.]]

In [[chemistry]], a '''solution''' is defined by [[IUPAC]] as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes. When, as is often but not necessarily the case, the sum of the mole fractions of solutes is small compared with unity, the solution is called a dilute solution. A superscript attached to the ∞ symbol for a property of a solution denotes the property in the limit of infinite dilution."<ref>{{cite web |title=Solution |url=https://goldbook.iupac.org/terms/view/S05746 |website=IUPAC Gold Book}}</ref> 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. The term "[[aqueous solution]]" is used when one of the solvents is [[water]].<ref name="auto">{{cite web|url=http://www.chemistry.wustl.edu/~coursedev/Online%20tutorials/Solutions.htm|title=Solutions|website=Washington University Chemistry Department|publisher=Washington University|access-date=13 April 2018}}</ref>

== Types ==
''Homogeneous'' means that the components of the mixture form a single phase. ''Heterogeneous'' means that the components of the mixture are of different phase. The properties of the mixture (such as concentration, temperature, and density) can be uniformly distributed through the volume but only in absence of diffusion phenomena or after their completion. Usually, the substance present in the greatest amount is considered the solvent. Solvents can be gases, liquids, or solids. One or more components present in the solution other than the solvent are called solutes. The solution has the same [[State of matter|physical state]] as the solvent.

=== 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 [[mixture]]s of gases. The [[Brownian motion]] and the permanent molecular agitation of gas molecules guarantee the homogeneity of the gaseous systems. Non-condensable gaseous mixtures (e.g., air/CO<sub>2</sub>, or air/xenon) do not spontaneously demix, nor sediment, as distinctly stratified and separate gas layers as a function of their [[relative density]]. [[Diffusion]] forces efficiently counteract [[gravity|gravitation]] forces under normal conditions prevailing on Earth. The case of condensable vapors is different: once the [[saturation vapor pressure]] at a given temperature is reached, vapor excess condenses into the [[liquid state]].

=== Liquid solutions ===
[[Liquid]]s dissolve gases, other liquids, and solids.  An example of a dissolved gas is [[oxygen]] in water, which allows fish to breathe under water.  An examples of a dissolved liquid is ethanol in water, as found in [[alcoholic beverage]]s.  An example of a dissolved solid is sugar water, which contains dissolved [[sucrose]].

=== Solid solutions ===
If the solvent is a [[solid]], then gases, liquids, and solids can be dissolved.
* 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 (element)|Mercury]] in [[gold]], forming an [[amalgam (chemistry)|amalgam]]
** Water in solid salt or [[sugar]], forming moist solids
** [[Hexane]] in [[paraffin wax]]
** [[Polymer]]s containing [[plasticizer]]s such as [[Bis(2-ethylhexyl) phthalate|phthalate]] (liquid) in [[Polyvinyl chloride|PVC]] (solid)
* Solid in solid:
** [[Steel]], basically a solution of carbon atoms in a crystalline matrix of iron atoms{{clarify|this is a gross oversimplification of the iron/iron carbide phase diagram|date=August 2021}} 
** [[Alloy]]s like [[bronze]] and many others
** [[Radium]] [[sulfate]] dissolved in [[barium sulfate]]: a true solid solution of Ra in BaSO<sub>4</sub>

== Solubility ==
{{Main article|Solubility|Solvation}}
The ability of one [[Chemical compound|compound]] to dissolve in another compound is called [[solubility]].{{clarify|what if one or both of the components is not a compound?|date=August 2021}} 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''.

All solutions have a positive [[entropy]] of mixing. The interactions between different molecules or ions may be energetically favored or not. If interactions are unfavorable, then the [[Thermodynamic free energy|free energy]] decreases with increasing solute concentration. At some point, the energy loss outweighs the entropy gain, and no more solute particles{{clarify|kind of particles|date=August 2021}} can be dissolved; the solution is said to be [[Saturated solution|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 solubilities that decrease with increased temperature. Such behavior is a result of an [[exothermic]] [[enthalpy of solution]]. Some [[surfactant]]s exhibit this behaviour. The solubility of liquids in liquids is generally less temperature-sensitive than that of solids or gases.

== Properties ==

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]], [[volume fraction]], and [[mole fraction]].

The properties of [[ideal solution]]s 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).

== Liquid solution characteristics==

{{See also|Solvent#Solvent classifications}}

In principle, all types of liquids can behave as solvents: liquid [[noble gas]]es, molten metals, molten salts, molten covalent networks, and molecular liquids. In the practice of chemistry and biochemistry, most solvents are molecular liquids. They can be classified into [[chemical polarity|polar and non-polar]], according to whether their molecules possess a permanent [[electric dipole moment]]. Another distinction is whether their molecules can form [[hydrogen bond]]s ([[protic]] and aprotic solvents). [[Water]], the most commonly used solvent, is both polar and sustains hydrogen bonds.

[[File:3D model hydrogen bonds in water.svg|thumb|150px|[[Water]] is a good solvent for some polar materials because water molecules are polar and capable of forming hydrogen bonds.]]

Salts dissolve in polar solvents, forming positive and negative ions that are attracted to the negative and positive ends of the solvent molecule, respectively. If the solvent is water, [[Solvation|hydration]] occurs when the charged solute ions become surrounded by water molecules. A standard example is aqueous saltwater. Such solutions are called [[electrolyte]]s. Whenever salt dissolves in water [[ion association]] has to be taken into account.

Polar solutes dissolve in polar solvents, forming polar bonds or hydrogen bonds. As an example, all alcoholic beverages are [[aqueous solution]]s of [[ethanol]]. On the other hand, non-polar solutes dissolve better in non-polar solvents. Examples are hydrocarbons such as [[oil]] and [[petroleum|grease]] that easily mix, while being incompatible with water.

An example of the immiscibility of oil and water is a leak of petroleum from a damaged tanker, that does not dissolve in the ocean water but rather floats on the surface.

== See also ==
{{Wiktionary|solution|solute}}
* {{annotated link|Molar solution}}
* {{annotated link|Percentage solution (disambiguation)}}
* {{annotated link|Solubility equilibrium}}
* {{annotated link|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 the evaporation of water from a sample.
* {{annotated link|Upper critical solution temperature}}
* {{annotated link|Lower critical solution temperature}}
* {{annotated link|Coil–globule transition}}

==References==
{{Reflist}}
*{{GoldBookRef|title=solution|file=S05746}}

==External links==
*{{Commonscatinline|Solutions}}

{{Chemical solutions|state=}}
{{Dosage forms|state=}}
{{Authority control}}

[[Category:Solutions| ]]
[[Category:Homogeneous chemical mixtures]]
[[Category:Alchemical processes]]
[[Category:Physical chemistry]]
[[Category:Colloidal chemistry]]
[[Category:Drug delivery devices]]
[[Category:Dosage forms]]