Editing Nitric acid (section)

== Physical and chemical properties ==
Commercially available nitric acid is an [[azeotrope]] with water at a concentration of 68% {{chem2|HNO3}}. This solution has a boiling temperature of {{cvt|120.5|C|F}} at {{cvt|1|atm|kPa psi}}. It is known as "concentrated nitric acid". The azeotrope of nitric acid and water is a colourless liquid at room temperature.

Two solid hydrates are known: the monohydrate {{chem2|HNO3*H2O}} or oxonium nitrate {{chem2|[H3O]+[NO3]-}} and the trihydrate {{chem2|HNO3*3H2O}}.

An older density scale is occasionally seen, with concentrated nitric acid specified as 42&nbsp;[[Baumé scale|Baumé]].<ref>{{Cite book| last = Dean| first = John| title = Lange's Handbook of Chemistry| edition = 14| publisher = [[McGraw-Hill]]| year = 1992| pages = [https://archive.org/details/langeshandbookof00lang_0/page/2 2.79–2.80]| isbn = 978-0-07-016194-8| url = https://archive.org/details/langeshandbookof00lang_0/page/2}}</ref>

===Contamination with nitrogen dioxide===
[[File:Fuming nitric acid 40ml.jpg|thumb|left|upright=0.9|Fuming nitric acid contaminated with yellow nitrogen dioxide]]
Nitric acid is subject to [[Heat|thermal]] or light decomposition and for this reason it was often stored in brown glass bottles:

{{block indent|{{chem2|4 HNO3 → 2 H2O + 4 NO2 + O2}}}}

This reaction may give rise to some non-negligible variations in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.

The nitrogen dioxide ({{chem2|NO2}}) and/or dinitrogen tetroxide ({{chem2|N2O4}}) remains dissolved in the nitric acid coloring it yellow or even red at higher temperatures. While the pure acid tends to give off white fumes when exposed to air, acid with dissolved nitrogen dioxide gives off reddish-brown vapors, leading to the common names "red fuming nitric acid" and "white fuming nitric acid". Nitrogen oxides ({{chem2|NO_{''x''}|}}) are soluble in nitric acid.

===Fuming nitric acid===
{{main|Red fuming nitric acid}}
Commercial-grade fuming nitric acid contains 98% {{chem2|HNO3}} and has a density of 1.50&nbsp;g/cm<sup>3</sup>. This grade is often used in the explosives industry. It is not as volatile nor as corrosive as the anhydrous acid and has the approximate concentration of 21.4&nbsp;M.

[[Red fuming nitric acid]], or RFNA, contains substantial quantities of dissolved nitrogen dioxide ({{chem2|NO2}}) leaving the solution with a reddish-brown color. Due to the dissolved nitrogen dioxide, the density of red fuming nitric acid is lower at 1.490&nbsp;g/cm<sup>3</sup>.

An ''inhibited'' fuming nitric acid, either white inhibited fuming nitric acid (IWFNA), or red inhibited fuming nitric acid (IRFNA), can be made by the addition of 0.6 to 0.7% [[hydrogen fluoride]] (HF). This fluoride is added for [[corrosion resistance]] in metal tanks. The fluoride creates a metal fluoride layer that protects the metal.

===Anhydrous nitric acid===
White fuming nitric acid, pure nitric acid or WFNA, is very close to anhydrous nitric acid. It is available as 99.9% nitric acid by assay, or about 24&nbsp;[[molarity|molar]]. One specification for white fuming nitric acid is that it has a maximum of 2% water and a maximum of 0.5% dissolved {{chem2|NO2}}. [[Anhydrous]] nitric acid is a colorless, low-[[viscosity]] (mobile) liquid with a density of 1.512&ndash;3&nbsp;g/cm<sup>3</sup> that solidifies at {{convert|-42|C}} to form white crystals.<ref>{{cite web |url=https://www.sigmaaldrich.com/US/en/product/mm/100455 |title=Nitric acid fuming 100% |publisher=Sigma-Aldrich |access-date=May 15, 2025}}</ref> Its dynamic viscosity under standard conditions is 0.76&nbsp;mPa·s.<ref>[https://www.wolframalpha.com/input?i=nitric+acid+viscosity "nitric acid viscosity"]. Wolfram Alpha Knowledgebase (2002). Champaign, Illinois.</ref> As it decomposes to {{chem2|NO2}} and water, it obtains a yellow tint. It boils at {{convert|83|C}}. It is usually stored in a glass shatterproof amber bottle with twice the volume of head space to allow for pressure build up, but even with those precautions the bottle must be vented monthly to release pressure.

===Structure and bonding===
[[Image:Nitric acid resonance.svg|thumb|upright=1.2|Two major resonance representations of {{chem2|HNO3}}]]
The two terminal N–O bonds are nearly equivalent and relatively short, at 1.20 and 1.21 Å.<ref name="CEAF">{{ cite journal | title = Microwave spectrum of DNO<sub>3</sub>, and average structures of nitric and nitrous acids | first1 = A. P. | last1 = Cox | first2 = M. C. | last2 = Ellis | first3 = C. J. | last3 = Attfield | first4 = A. C. | last4 = Ferris | journal = [[Journal of Molecular Structure]] | year = 1994 | volume = 320 | issue = 1–2 | pages = 91–106 | doi = 10.1016/0022-2860(93)08008-R | bibcode = 1994JMoSt.320...91C }}</ref> This can be explained by theories of [[Resonance (chemistry)|resonance]]; the two major [[Resonance (chemistry)|canonical forms]] show some [[double bond]] character in these two bonds, causing them to be shorter than N–O [[single bond]]s. The third N–O bond is elongated because its O atom is bonded to H atom,<ref>{{cite journal |first=V. |last=Luzzati |title=Structure cristalline de l'acide nitrique anhydre |language=fr |journal=Acta Crystallographica |year=1951 |volume=4 |issue= 2|pages=120–131 |doi=10.1107/S0365110X51000404|bibcode=1951AcCry...4..120L |doi-access=free }}</ref><ref name=Allan>{{cite journal |first1=D. R. |last1=Allan |first2=W. G. |last2=Marshall |first3=D. J. |last3=Francis |first4=I. D. H. |last4=Oswald |first5=C. R. |last5=Pulham |first6=C. |last6=Spanswick |title=The crystal structures of the low-temperature and high-pressure polymorphs of nitric acid |journal=Dalton Transactions |year=2010 |volume=39 |issue=15 |pages=3736–3743 |doi=10.1039/B923975H |pmid=20354626 |url=https://strathprints.strath.ac.uk/26164/1/The_crystal_structures_of_the_low-temperature_and_high-pressure_polymorphs.pdf |type=Submitted manuscript }}</ref> with a [[bond length]] of 1.41 Å in the gas phase.<ref name="CEAF" /> The molecule is slightly aplanar (the [[nitro group|{{chem2|NO2}}]] and NOH planes are tilted away from each other by 2°) and there is [[Conformational isomerism|restricted rotation]] about the N–OH single bond.<ref name="G&E" /><ref>{{cite journal | title = Microwave Spectrum and Structure of Nitric Acid | first1 = A. P. | last1 = Cox | first2 = J. M. | last2 = Riveros | journal = The Journal of Chemical Physics | year = 1965 | volume = 42 | issue = 9 | page = 3106 | doi = 10.1063/1.1696387 | bibcode = 1965JChPh..42.3106C }}</ref>