Editing Silicon dioxide (section)

==Chemical reactions==
Silicon dioxide is a relatively inert material (hence its widespread occurrence as a mineral). Silica is often used as inert containers for chemical reactions. At high temperatures, it is  converted to silicon by reduction with carbon.

[[Fluorine]] reacts with silicon dioxide to form [[Silicon tetrafluoride|SiF<sub>4</sub>]] and O<sub>2</sub> whereas the other halogen gases (Cl<sub>2</sub>, Br<sub>2</sub>, I<sub>2</sub>) are unreactive.<ref name="Greenwood"/>

Most forms of silicon dioxide are attacked ("etched") by [[hydrofluoric acid]] (HF) to produce [[hexafluorosilicic acid]]:<ref name="Wiberg&Holleman"/>
:{{chem2|SiO2 + 6 HF -> H2SiF6 + 2 H2O}}
[[Stishovite]] does not react to HF  to any significant degree.<ref name="Fleischer1962">{{cite journal|last1=Fleischer|first1=Michael|year=1962|title=New mineral names|journal=American Mineralogist|volume=47|issue=2|pages=172–174|publisher=Mineralogical Society of America|url=http://rruff.info/uploads/AM47_805.pdf |archive-url=https://web.archive.org/web/20110722000427/http://rruff.info/uploads/AM47_805.pdf |archive-date=2011-07-22 |url-status=live}}</ref> 
HF is used to remove or pattern silicon dioxide in the semiconductor industry.

Silicon dioxide acts as a [[Acid–base reaction#Lux–Flood definition|Lux–Flood acid]], being able to react with bases under certain conditions. As it does not contain any hydrogen, non-hydrated silica cannot directly act as a [[Brønsted–Lowry acid–base theory|Brønsted–Lowry acid]]. While silicon dioxide is only poorly soluble in water at low or neutral [[pH]] (typically, 2 × 10<sup>−4</sup> [[Molar concentration|M]] for [[quartz]] up to 10<sup>−3</sup> [[Molar concentration|M]] for [[cryptocrystalline]] [[chalcedony]]), strong bases react with glass and easily dissolve it. Therefore, strong bases have to be stored in plastic bottles to avoid jamming the bottle cap, to preserve the integrity of the recipient, and to avoid undesirable contamination by silicate anions.<ref name="Rodgers2011">{{cite book|url=https://books.google.com/books?id=BY8IAAAAQBAJ&pg=PA421|title=Descriptive Inorganic, Coordination, and Solid State Chemistry|vauthors=Rodgers GE|publisher=Cengage Learning|year=2011|isbn=9781133172482|pages=421–2}}</ref>

Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:<ref name="Greenwood"/>
:<chem>SiO2 + 2 NaOH -> Na2SiO3 + H2O</chem>

Silicon dioxide will neutralise basic metal oxides (e.g. [[sodium oxide]], [[potassium oxide]], [[lead(II) oxide]], [[zinc oxide]], or mixtures of oxides, forming [[silicate]]s and glasses as the Si-O-Si bonds in silica are broken successively).<ref name="Wiberg&Holleman"/> As an example the reaction of sodium oxide and SiO<sub>2</sub> can produce sodium [[orthosilicate]], sodium silicate, and glasses, dependent on the proportions of reactants:<ref name = "Greenwood"/>
:<chem>2 Na2O + SiO2 -> Na4SiO4;</chem>

:<chem>Na2O + SiO2 -> Na2SiO3;</chem>

:<math>(0.25-0.8)</math> <chem>Na2O + SiO2 -> glass</chem>.

Examples of such glasses have commercial significance, e.g. [[soda–lime glass]], [[borosilicate glass]], [[lead glass]]. In these glasses, silica is termed the network former or lattice former.<ref name="Wiberg&Holleman"/> The reaction is also used in [[blast furnace]]s to remove sand impurities in the ore by neutralisation with [[calcium oxide]], forming [[calcium silicate]] [[slag]].
[[File:Fibreoptic.jpg|thumb|Bundle of [[optical fibre]]s composed of high purity silica]]

Silicon dioxide reacts in heated [[reflux]] under [[dinitrogen]] with [[ethylene glycol]] and an [[alkali metal]] base to produce highly reactive, [[Hypervalent molecule#Pentacoordinated silicon|pentacoordinate]] silicates which provide access to a wide variety of new silicon compounds.<ref name=Laine>{{cite journal|last1=Laine|first1=Richard M.|last2=Blohowiak|first2=Kay Youngdahl|last3=Robinson|first3=Timothy R.|last4=Hoppe|first4=Martin L.|last5=Nardi|first5=Paola|last6=Kampf|first6=Jeffrey|last7=Uhm|first7=Jackie|title=Synthesis of pentacoordinate silicon complexes from SiO<sub>2</sub>|journal=Nature|volume=353|date=17 October 1991|issue=6345|pages=642–644|doi=10.1038/353642a0|bibcode=1991Natur.353..642L|url=https://deepblue.lib.umich.edu/bitstream/2027.42/62810/1/353642a0.pdf |archive-url=https://web.archive.org/web/20170819150753/http://deepblue.lib.umich.edu/bitstream/2027.42/62810/1/353642a0.pdf |archive-date=2017-08-19 |url-status=live|hdl=2027.42/62810|s2cid=4310228|hdl-access=free}}</ref> The silicates are essentially insoluble in all [[Solvent|polar solvent]] except [[methanol]].

Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO:<ref name="Wiberg&Holleman"/>
:<chem>SiO2 + Si -> 2 SiO</chem>

===Water solubility===
The solubility of silicon dioxide in water strongly depends on its crystalline form and is three to four times higher for amorphous silica than quartz; as a function of temperature, it peaks around {{convert|340|°C}}.<ref>{{cite journal|vauthors=Fournier RO, Rowe JJ|year=1977|title=The solubility of amorphous silica in water at high temperatures and high pressures|url=http://www.minsocam.org/ammin/AM62/AM62_1052.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.minsocam.org/ammin/AM62/AM62_1052.pdf |archive-date=2022-10-10 |url-status=live|journal=[[American Mineralogist|Am. Mineral.]]|volume=62|pages=1052–1056}}</ref> This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel that is cooler at the top. Crystals of 0.5–1&nbsp; kg can be grown for 1–2 months.<ref name="Wiberg&Holleman">{{Holleman&Wiberg}}</ref> These crystals are a source of very pure quartz for use in electronic applications.<ref name="Greenwood"/> Above the [[critical point (thermodynamics)|critical temperature]] of water {{convert|647.096|K}} and a pressure of {{convert|22.064|MPa}} or higher, water is a [[supercritical fluid]] and solubility is once again higher than at lower temperatures.<ref>{{cite journal | url=https://ui.adsabs.harvard.edu/abs/2019EGUGA..21.4614O/abstract | bibcode=2019EGUGA..21.4614O | title=Formation of silica particles from supercritical fluids and its impacts on the hydrological properties in the crust | last1=Okamoto | first1=Atsushi | journal=EGU General Assembly Conference Abstracts | year=2019 | page=4614 }}</ref>