Editing Hydrate (section)

===Inorganic chemistry===
{{main|Water of crystallization}}
Hydrates are not 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.