View source for Hydrate

{{short description|Substance containing water or its constituent elements}}
{{about|chemical compounds|hydration in humans or animals|Drinking}}
In [[chemistry]], a '''hydrate''' is a substance that contains [[water]] or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understood.

==Chemical nature==

===Inorganic chemistry===
{{main|Water of crystallization}}
Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the [[crystal]]"<ref>{{Greenwood&Earnshaw2nd|page=625}}</ref> that are either bound to a metal center or that have crystallized with the metal complex.  Such hydrates are also said to contain ''[[water of crystallization]]'' or ''water of hydration''.  If the water is [[heavy water]] in which the constituent hydrogen is the [[isotope]] [[deuterium]], then the term ''deuterate'' may be used in place of ''hydrate''.<ref>{{cite book | title = Displacement of Water and Its Control of Biochemical Reactions | page = 299 | author = Sherry Lewin | publisher = Academic Press Inc. (London) Ltd. | isbn = 0124462502 | url = https://archive.org/details/displacementofwa0000lewi/page/298/mode/2up}}</ref><ref>{{cite journal | title = A Name and Symbol for H² | author1 = [[Harold C. Urey]] | author2= [[G. M. Murphy]] | author3 = [[Ferdinand Brickwedde|F. G. Brickwedde]] | year = 1933 | journal = Journal of Chemical Physics | doi = 10.1063/1.1749326 | volume =1 | pages = 512–513}}</ref>

{|style="float:right; margin: 0 0 1em 1em; border:1px solid black;"
| [[File:Cobalt(II) chloride.jpg|190px]] || [[File:Cobalt(II)-chloride-hexahydrate-sample.jpg|190px]]
|- align="center"
| [[Anhydrous]]<br /> cobalt(II) chloride<br />'''{{chem2|CoCl2}}''' (blue)|| Cobalt(II) chloride<br /> hexahydrate<br />'''{{chem2|CoCl2*6H2O}}''' (pink)
|}
A colorful example is [[cobalt(II) chloride]], which turns from blue to red upon [[mineral hydration|hydration]], and can therefore be used as a water indicator.

The notation "''hydrated compound'''''⋅''n''{{chem2|H2O}}'''", where ''n'' is the number of water molecules per [[formula unit]] of the salt, is commonly used to show that a salt is hydrated.  The ''n'' is usually a low [[integer]], though it is possible for fractional values to occur. For example, in a '''monohydrate''' ''n''&nbsp;=&nbsp;1, and in a '''hexahydrate''' ''n''&nbsp;=&nbsp;6. Numerical  prefixes mostly of Greek origin are:<ref>[http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf Nomenclature of Inorganic Chemistry] {{Webarchive|url=https://web.archive.org/web/20180709210050/http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf |date=2018-07-09 }}. IUPAC Recommendations 2005. Table IV Multiplicative Prefixes, p.&nbsp;258.</ref>
{{columns-list|colwidth=8em|
* Hemi – 0.5
* Mono – 1
* Sesqui – 1.5
* Di – 2
* Tri – 3
* Tetra – 4
* Penta – 5
* Hexa – 6
* Hepta – 7
* Octa – 8
* Nona – 9
* Deca – 10
* Undeca – 11
* Dodeca – 12
* Trideca – 13
* Tetradeca – 14
}}

A hydrate that has lost water is referred to as an [[Inorganic anhydride|anhydride]]; the remaining water, if any exists, can only be removed with very strong heating. A substance that does not contain any water is referred to as [[anhydrous]]. Some anhydrous compounds are hydrated so easily that they are said to be [[hygroscopic]] and are used as drying agents or [[desiccant]]s.

===Organic chemistry===
{{For|hydrates of carbonyl compounds|geminal diol}}
In organic chemistry, a hydrate is a compound formed by the hydration, i.e. "Addition of water or of the elements of water (i.e. H and OH) to a molecular entity".<ref>{{GoldBookRef|title=Hydration|file=H02876|doi=10.1351/goldbook.H02876|year=2019|accessdate=27 July 2020}}</ref> For example: [[ethanol]], {{chem2|CH3\sCH2\sOH}}, is the product of the [[hydration reaction]] of [[ethene]], {{chem2|CH2\dCH2}}, formed by the addition of H to one C and OH to the other C, and so can be considered as the hydrate of ethene.  A molecule of water may be eliminated, for example, by the action of [[sulfuric acid]]. Another example is [[chloral hydrate]], {{chem2|CCl3\sCH(OH)2}}, which can be formed by reaction of water with [[chloral]], {{chem2|CCl3\sCH\dO}}.

Many organic molecules, as well as inorganic molecules, form crystals that incorporate water into the crystalline structure without chemical alteration of the organic molecule ([[water of crystallization]]). The sugar [[trehalose]], for example, exists in both an [[anhydrous]] form ([[melting point]] 203&nbsp;°C) and as a dihydrate (melting point 97&nbsp;°C). [[Protein crystallization|Protein crystals]] commonly have as much as 50% water content.

Molecules are also labeled as hydrates for historical reasons not covered above. [[Glucose]], {{chem2|C6H12O6}}, was originally thought of as {{chem2|C6(H2O)6}} and described as a [[carbohydrate]].

Hydrate formation is common for [[active ingredient]]s. Many manufacturing processes provide an opportunity for hydrates to form and the state of hydration can be changed with environmental humidity and time. The state of hydration of an active pharmaceutical ingredient can significantly affect the solubility and dissolution rate and therefore its [[bioavailability]].<ref>Surov, Artem O., Nikita A. Vasilev, Andrei V. Churakov, Julia Stroh, Franziska Emmerling, and German L. Perlovich. "Solid Forms of Ciprofloxacin Salicylate: Polymorphism, Formation Pathways and Thermodynamic Stability". Crystal Growth & Design (2019). {{doi|10.1021/acs.cgd.9b00185}}.</ref>

===Clathrate hydrates===
{{main|Clathrate hydrate}}
[[Clathrate hydrates]] (also known as gas hydrates, gas clathrates, etc.) are water ice with gas molecules trapped within; they are a form of [[clathrate]]. An important example is  [[methane hydrate]] (also known as gas hydrate, methane clathrate, etc.).

Nonpolar molecules, such as methane, can form [[clathrate hydrate]]s with water, especially under high pressure. Although there is no [[hydrogen bond]]ing between water and guest molecules when methane is the guest molecule of the clathrate, guest–host hydrogen bonding often forms when the guest is a larger organic molecule such as [[tetrahydrofuran]]. In such cases, the guest–host hydrogen bonds result in the formation of L-type [[Bjerrum defect]]s in the clathrate lattice.<ref>{{cite journal |author=Alavi S. |author2=Susilo R. |author3=Ripmeester J. A. |title=Linking microscopic guest properties to macroscopic observables in clathrate hydrates: guest-host hydrogen bonding |journal=The Journal of Chemical Physics |volume=130 |issue=17 |year=2009 |page=174501 |format=PDF |url=http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JCPSA6000130000017174501000001&bypassSSO=1 |doi=10.1063/1.3124187 |pmid=19425784 |bibcode=2009JChPh.130q4501A |access-date=2010-09-09 |archive-date=2020-04-13 |archive-url=https://web.archive.org/web/20200413233622/http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JCPSA6000130000017174501000001&bypassSSO=1 |url-status=live |url-access=subscription }}</ref><ref>{{Cite journal|doi=10.1039/C8CS00989A|title=Gas hydrates in sustainable chemistry|year=2020|last1=Hassanpouryouzband|first1=Aliakbar|last2 = Joonaki|first2 = Edris|last3 = Vasheghani Farahani|first3 = Mehrdad|last4 = Takeya|first4 = Satoshi|last5 = Ruppel|first5 = Carolyn|last6 = Yang|first6 = Jinhai|last7 = J. English|first7 = Niall|last8 = M. Schicks|first8 = Judith|last9 = Edlmann|first9 = Katriona|last10 = Mehrabian|first10 = Hadi|last11 = M. Aman|first11 = Zachary|last12 = Tohidi|first12 = Bahman|journal=Chemical Society Reviews|volume=49|issue=15|pages=5225–5309|pmid=32567615|s2cid=219971360|doi-access = free|hdl = 1912/26136|hdl-access = free}}</ref>

==Stability==
The stability of hydrates is generally determined by the nature of the compounds, their temperature, and the [[relative humidity]] (if they are exposed to air).

== See also ==
{{div col|colwidth=20em}}
* [[Efflorescence]]
* [[Hygroscopy]]
* [[Mineral hydration]]
* [[Water of crystallization]]
* [[Hemihydrate]]
*[[Hydrous oxide]]
{{div col end}}

== References ==
{{reflist|25em}}

{{Authority control}}

[[Category:Hydrates]]