Nitrogen Dioxide
Properties
| State | Gas at room temperature (above 21.1 °C) |
| Color | Reddish-brown |
| Solubility | Reacts with water to form nitric acid and nitrous acid |
| Melting Point | -11.2 °C |
| Boiling Point | 21.1 °C |
About Nitrogen Dioxide
Nitrogen dioxide is the brown gas of chemistry — formula NO2, molar mass 46.005 g/mol — and it's visible to the unaided eye at concentrations above about 1 ppm, which is what makes it a teaching favorite and a regulatory headache simultaneously. The brown color comes from a broad absorption band peaking near 400 nm that extends across blue and green wavelengths; what your eye sees is the transmitted red and yellow component. NO2 is a paramagnetic open-shell radical with one unpaired electron on nitrogen (17 valence electrons total), and it dimerizes reversibly to colorless N2O4 in a temperature-dependent equilibrium: 2NO2 ⇌ N2O4, with ΔH ≈ -57 kJ/mol. Sealed tubes containing this equilibrium go brown at room temperature, fade to colorless when chilled in ice, and darken again when warmed — the cleanest demonstration of Le Chatelier's principle most students ever see. Atmospherically, NO2 is a key urban air pollutant: vehicle engines and combustion power plants generate NO at high temperatures, which oxidizes to NO2 in the atmosphere. NO2 then drives photochemical smog through NO2 + hv → NO + O followed by O + O2 → O3, building ground-level ozone. It also reacts with water vapor to form HNO3, contributing to acid rain. Industrially, NO2 (and its dimer N2O4) is a key intermediate in the Ostwald nitric acid process and serves as the storable oxidizer in hypergolic rocket propellant pairs with hydrazines.
Where you'll encounter it
If you've ever pulled the cap off a bottle of old concentrated nitric acid and seen a brown plume curl out, that's NO2 — photodecomposition of HNO3 generates NO2 over weeks of storage. In a teaching demo, the standard prep is to drop copper turnings into concentrated HNO3 in a stoppered flask: the Cu metal reduces nitrate to NO2, the flask fills with characteristic brown gas, and the demonstration becomes a Le Chatelier exercise when the flask is plunged into ice and the brown fades visibly. Atmospheric chemists track urban NO2 plumes by satellite — TROPOMI on Sentinel-5P maps the brown signature globally, and during the COVID-19 lockdowns of spring 2020, NO2 columns over major Chinese and European cities dropped 30-50% within weeks, providing one of the cleanest proofs ever of the link between traffic emissions and air quality.
Common Uses
- Intermediate in the Ostwald process for nitric acid manufacture (NH3 → NO → NO2 → HNO3)
- Storable hypergolic oxidizer (as N2O4) paired with hydrazines in spacecraft attitude-control thrusters
- Reference gas standard for chemiluminescence NOx analyzers in air-quality monitoring stations
- Le Chatelier's principle demonstration via the reversible 2NO2 ⇌ N2O4 equilibrium in sealed tubes
- Selective oxidant in organic synthesis for nitration of activated aromatics and α-oxidation of ketones
- Tracer gas for satellite measurement of fossil-fuel combustion plumes (TROPOMI, OMI)
- Sterilization agent (NO2 fogging) for spacecraft and biosafety facility decontamination
Safety Information
GHS classifications: H270 (may cause or intensify fire; oxidizer), H280 (gas under pressure), H314 (causes severe skin burns and eye damage), H330 (fatal if inhaled). Occupational exposure limits for NO2 are among the lowest set for any industrial gas, reflecting how dangerous it is. The signature toxicity is delayed pulmonary edema: NO2 dissolves on lung surfaces forming HNO3 that damages alveolar tissue, but symptoms can be minimal at the time of exposure and lethal pulmonary edema develops 6-24 hours later (silo-filler's disease — fermenting silage in farm silos generates NO2 that has killed many farmers who entered the silo too early). Always handle in a fume hood with NO2-specific monitoring; respirators must be supplied-air, not cartridge. Reacts violently with organic materials and reducing agents.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.