Editing Potassium nitrate (section)

==Properties==
Potassium nitrate has an [[orthorhombic]] crystal structure at room temperature,<ref name="Adiwidjaja&Pohl">{{cite journal | title = Superstructure of α-phase potassium nitrate | first1 = G. | last1 = Adiwidjaja | first2 = D. | last2 = Pohl | journal = [[Acta Crystallographica Section C]] | year = 2003 | volume = 59 | issue = 12 | pages = i139–i140 | doi = 10.1107/S0108270103025277 | pmid = 14671340 | bibcode = 2003AcCrC..59I.139A}}</ref> which transforms to a trigonal system at {{convert|128|C|F}}. On cooling from {{convert|200|C|F}}, another trigonal phase forms between {{convert|124|C|F}} and {{convert|100|C|F}}.<ref>{{cite journal | title = The crystal structures of γ- and β-KNO<sub>3</sub> and the α ← γ ← β phase transformations | first1 = J. K. | last1 = Nimmo | first2 = B. W. | last2 = Lucas | journal = [[Acta Crystallographica Section B]] | year = 1976 | volume = 32 | issue = 7 | pages = 1968–1971 | doi = 10.1107/S0567740876006894 | doi-access = free}}</ref><ref>{{cite journal | title = Growth and single-crystal refinement of phase-III potassium nitrate, KNO<sub>3</sub> | first1 = E. J. | last1 = Freney | first2 = L. A. J. | last2 = Garvie | first3 = T. L. | last3 = Groy | first4 = P. R. | last4 = Buseck | journal = [[Acta Crystallographica Section B]] | year = 2009 | volume = 65 | issue = 6 | pages = 659–663 | doi = 10.1107/S0108768109041019 | pmid = 19923693}}</ref>

[[Sodium nitrate]] is isomorphous with [[calcite]], the most stable form of [[calcium carbonate]], whereas room-temperature potassium nitrate is [[isomorphous]] with [[aragonite]], a slightly less stable [[Polymorphism (materials science)|polymorph]] of calcium carbonate. The difference is attributed to the similarity in size between [[nitrate]] ({{chem2|NO3-}}) and [[carbonate]] ({{chem2|CO3(2-)}}) ions and the fact that the potassium ion ({{chem2|K+}}) is larger than sodium ({{chem2|Na+}}) and calcium ({{chem2|Ca(2+)}}) ions.<ref name="G&E">{{Greenwood&Earnshaw2nd|page=407}}</ref>

In the room-temperature structure of potassium nitrate, each potassium ion is surrounded by 6 nitrate ions. In turn, each nitrate ion is surrounded by 6 potassium ions.<ref name="Adiwidjaja&Pohl" />

{| class="wikitable" style="margin:1em auto; text-align:center;"
|+Room temperature [[crystal structure]] and [[coordination geometry]] of potassium nitrate<ref name="Adiwidjaja&Pohl" />
|-
! Unit cell
! Potassium coordination
! Nitrate coordination
|-
| [[File:Potassium-nitrate-superstructure-unit-cell-3D-bs-17.png|250px]]
| [[File:Potassium-nitrate-xtal-K1-coord-3D-bs-17.png|200px]]
| [[File:Potassium-nitrate-xtal-N1-coord-3D-bs-17.png|150px]]
|}

Potassium nitrate is moderately soluble in water, but its solubility increases with temperature. The aqueous solution is almost neutral, exhibiting [[pH]] 6.2 at {{convert|14|C|F}} for a 10% solution of commercial powder. It is not very [[hygroscopic]], absorbing about 0.03% water in 80% [[relative humidity]] over 50 days. It is insoluble in alcohol and is not poisonous; it can react explosively with [[reducing agents]], but it is not explosive on its own.<ref name=b1/>

===Thermal decomposition===
Between {{convert|550-790|C|F}}, potassium nitrate reaches a temperature-dependent equilibrium with [[potassium nitrite]]:<ref>{{cite journal|title=The Kinetics of the Thermal Decomposition of Potassium Nitrate and of the Reaction between Potassium Nitrite and Oxygen|author=Eli S. Freeman|journal=J. Am. Chem. Soc.|year=1957|volume=79|pages=838–842|doi=10.1021/ja01561a015|issue=4|bibcode=1957JAChS..79..838F }}</ref>
:{{chem2|2 KNO3 ⇌ 2 KNO2 + O2}}