CID 944
HNO3
Nitric acid
- H 1
- N 1
- O 3
Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.
- IUPAC name
- nitric acid
- Molar mass
- 63.0120(42) g/mol
- Atoms
- 5
- SMILES
- [N+](=O)(O)[O-]
- InChIKey
- GRYLNZFGIOXLOG-UHFFFAOYSA-N
- PubChem CID
- 944
As early as the eighth century, alchemists were using this liquid under the name aqua fortis, “strong water”. It earned its place because it dissolves silver but not gold, so the two metals could be parted. The name still appears among its synonyms, beside engravers acid.
OpenStax describes nitric acid as both a strong acid and a strong oxidising agent. Its molecule joins one nitrogen, three oxygen and one hydrogen atom, and a mole weighs 63.012 g. The nitrogen has an oxidation number of +5, a bookkeeping figure rather than a real charge.
Pure nitric acid is a colourless liquid that mixes with water in any proportion. A stored bottle often turns yellow or brown instead. The acid slowly decomposes, and the nitrogen dioxide it releases colours the liquid; light and heat both drive that breakdown. The same tendency makes the pure liquid very hard to prepare and keep.
Structure and bonding
In nitric acid, nitrogen sits at the centre with three oxygen atoms around it, and one of those oxygens also holds the single hydrogen. Turned edge-on, the model is flat: all five atoms lie in one plane in PubChem’s computed structure. Labels prints one N, three O and one H.
The drawing is one resonance form. It gives nitrogen a double bond to one outer oxygen and a single bond to the other, which carries a charge of −1 while nitrogen carries +1. OpenStax shows several resonance forms for nitric acid, and the real molecule is their average; it does not flip between them. In the model both outer N–O bonds measure 1.230 Å, while the bond to the OH oxygen is longer, at 1.398 Å.
In the Space-filling view the spheres merge into one flat outline, the small hydrogen a bump on one oxygen. The O–H bond is where the acidity lives: nitrogen at +5 pulls electrons away from that oxygen, weakening its hold on the hydrogen. The model shows the neutral molecule of the pure acid. Dissolved in water, nitric acid ionises completely into hydronium and nitrate ions.
Where it turns up
Traces form in the air after thunderstorms, and OpenStax counts nitric acid among the acids in acid rain. Lightning passing through air makes nitrogen react with oxygen to form nitrogen oxides, and one of them, nitrogen dioxide, reacting with water vapour gives the acid. Its salts are spread widely in nature, and huge deposits of Chile saltpetre, sodium nitrate, lie in the desert near the Chile–Peru border.
The chemistry laboratory knows it as the concentrated reagent, a solution of about 68 per cent acid. Chip makers use a 67 per cent solution as a wet etchant in semiconductor manufacturing. Photo-engraving, etching steel and metallurgy take a share. So do hobby crafts: enamelling, lithography and the chemical patinas applied to metal all appear in PubChem’s exposure lists. A consumer products database there counts four household products containing it.
Why it matters
Industry makes nitric acid from ammonia by the Ostwald process. Ammonia is oxidised to nitric oxide over a platinum catalyst, the nitric oxide goes on to nitrogen dioxide, and further oxidation with water gives the acid. US manufacturers reported 15 to 20 billion pounds for 2023.
Most of that is neutralised with ammonia to give ammonium nitrate, about 74 to 78 per cent of US production on one PubChem estimate. Nitrate salts are valued as fertilisers, and OpenStax adds preparing soluble metal nitrates as a major use. Its other main uses are explosives, nylon precursors, dyes, plastics and drugs.
Its behaviour with metals differs from the textbook acid pattern. On OpenStax’s account, nitric acid acting on a metal rarely releases more than a little hydrogen; the nitrogen is reduced instead. Products depend on the acid’s concentration, the metal and the temperature. Copper gives nitrogen dioxide with the concentrated acid and nitric oxide with dilute acid. PubChem’s hazard data say it releases hydrogen from most metals. Mercury, which cannot displace hydrogen from acids, still reacts with oxidising acids like this one.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Molecular formula (Hill order) | HNO3 | ||
| Molar mass | 63.0120(42) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Hydrogen atoms | 1 | ||
| Nitrogen atoms | 1 | ||
| Oxygen atoms | 3 | ||
| Bonds | 4 | ||
| Single bonds | 3 | ||
| Double bonds | 1 | ||
| PubChem molecular weight | 63.013 g/mol | g/mol | PubChem |
Safety
Nitric acid is corrosive and oxidising, and the European Chemicals Agency’s aggregate classification gives it the signal word Danger. Nearly every report says it causes severe skin burns and eye damage, and almost all say it may intensify fire. The EU’s harmonised classification rates it toxic if inhaled, and fatal if inhaled for the most concentrated grades. NIOSH names the eyes, skin, respiratory system and teeth as targets. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide entry are the authoritative sheets.
Questions
What does the nitric acid model show?
Five atoms joined by four bonds, all lying flat in one plane in PubChem's computed model. Nitrogen sits in the middle with three oxygens around it, and one of those oxygens holds the hydrogen. The drawing gives nitrogen a charge of +1 and one outer oxygen −1. That is a single resonance form; the real molecule is an average of its forms, and the two outer N–O bonds in the model are the same length.
Why does nitric acid turn yellow?
Pure nitric acid has no colour at all. Over time it slowly breaks down, and one product is nitrogen dioxide, NO2, which stays dissolved in the liquid and tints it yellow or brown. Light speeds the breakdown, and so does heat. The so-called fuming grade is concentrated acid that carries dissolved nitrogen dioxide, and the pure liquid is hard to keep for the same reason.
Is nitric acid a strong acid?
Yes. In water it ionises completely, handing its hydrogen ion to a water molecule to give hydronium, H3O+, and the nitrate ion, NO3−. Among the strong acids OpenStax lists, it is the weakest, with perchloric acid the strongest, although water hides the difference: hydronium is the strongest acid that can survive in it. Nitrous acid, HNO2, with nitrogen at +3 rather than +5, is weaker.
Does nitric acid dissolve gold?
Not on its own. That is exactly what made it useful to the alchemists: it dissolves silver and leaves gold behind, so it could part the two metals. Gold and platinum do dissolve in a mixture of concentrated nitric and hydrochloric acids called aqua regia, royal water. Nitric acid still has a working role here, since PubChem's manufacturing data list dissolving noble metals among its uses.
Made of the same elements
- HNO2 Nitrous acid 4 atoms, 3 bonds.
Appears in these articles
- An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.
Sources
- PubChem CID 944, nitric acid
- IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
- PubChem Laboratory Chemical Safety Summary for nitric acid (CID 944)
- NIOSH Pocket Guide to Chemical Hazards, Nitric acid
- OpenStax, Chemistry 2e, §18.9 Occurrence, Preparation, and Compounds of Oxygen
- OpenStax, Chemistry 2e, §18.7 Occurrence, Preparation, and Properties of Nitrogen
- OpenStax, Chemistry 2e, §14.3 Relative Strengths of Acids and Bases
- OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance
- OpenStax, Chemistry 2e, §8.3 Multiple Bonds
- OpenStax, Chemistry 2e, §18.1 Periodicity