Nitrogen Dioxide

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Nitrogen Dioxide


Nitrogen dioxide (NYE-truh-jin dye-OK-side) is a toxic reddish-brown gas or yellowish-brown liquid with a pungent, irritating odor. Above 21.15°C (70.07°F), it exists as the red-dish-brown gas. Below that temperature, it becomes the yellowish-brown liquid. When liquified under pressure, it forms a fuming brown liquid. The brown liquid is actually a mixture of nitrogen dioxide and dinitrogen tetroxide (N2O4), a dimeric form of nitrogen dioxide. A dimer is a molecule that consists of two identical molecules combined with each other. When cooled below −11.2°C (11.8°F), the liquid freezes to form a colorless crystalline solid that consists almost entirely of the dimeric form, N2O4.



Dinitrogen tetroxide; nitrogen peroxide




Nitrogen, oxygen


Nonmetallic oxide (inorganic)


Gas or liquid


46.01 g/mol


−11.2°C (11.8°F)


21.15°C (70.07°F)


Reacts with water to form nitric acid (HNO3) and nitrous acid (HNO2)

Small amounts of nitrogen dioxide are present naturally in the atmosphere as the result of lightning strikes, volcanic action, forest fires, and bacterial action on dead plants and animals. Much larger amounts are present because of human activities, primarily the combustion of fossil fuels, such as coal and petroleum products.


Nitrogen dioxide occurs naturally when nitric oxide (NO) is oxidized in the atmosphere. Nitric oxide forms naturally whenever sufficient energy is available to make possible the reaction between nitrogen and oxygen, the two primary components of the atmosphere. The formula for this reaction is

N2 + O2 → 2NO

Nitric oxide, in turn, reacts readily with oxygen to form nitrogen dioxide. This reaction can be shown as

2NO + O2 → 2NO2.

The amount of nitrogen dioxide produced naturally is so small that the gas's distinctive brown color is unnoticeable.

Such is not the case, however, in situations where nitric oxide and nitrogen dioxide are produced by human activities. The burning of coal or oil in plants that generate electricity and the combustion of gasoline in automobiles and trucks produce significant quantities of nitric oxide, which rapidly oxidizes to form nitrogen dioxide in the atmosphere. In such cases, sufficient nitrogen dioxide may be present to produce the yellowish hazy condition associated with smog.

Commercially, nitrogen dioxide is made by one of two processes, the decomposition of nitric acid (HNO3) or the oxidation of ammonia (NH3) gas. The gas can also be produced on a smaller scale or in the laboratory by a number of methods that involve the decomposition of the nitrate ion (NO3). For example, heating lead(II) nitrate [Pb(NO3)2] results in the formation of lead(II) oxide (PbO), nitrogen dioxide, and oxygen.


The most important uses of nitrogen dioxide are in the manufacture of nitric and sulfuric acids, two of the most widely used inorganic acids. The compound is also used widely as an oxidizing agent and nitrating agent. An oxidizing agent is a substance that makes oxygen available to other compounds in order to bring about a particular reaction. A nitrating agent is one that provides the nitro group (-NO2)in producing a new compound. As an example, adding nitrate groups to the organic compound known as toluene (C6H5CH3) converts it into trinitrotoluene (TNT), a powerful explosive. Nitrogen dioxide is also used in rockets, where it supplies the oxygen needed to burn the rocket fuel; as a catalyst in a number of industrial operations; in the bleaching of flour; and as a polymerization inhibitor in the manufacture of some plastics. A polymerization inhibitor is a substance that stops the formation of a polymer at some desired point in its production or use.

Interesting Facts

  • Scientists often use the formula NOx to refer to the oxides of nitrogen that occur in polluted air. The two most important of those oxides are nitric oxide (NO) and nitrogen dioxide.

Nitrogen dioxide poses both safety and health hazards. As a strong oxidizing agent, it reacts readily with combustible materials, such as paper, cloth, and other organic matter to produce fires or explosions. It is also a toxic material, producing some biological effects at relatively low concentrations in the air. These effects include irritation of the eyes, nose, and throat; coughing; congestion; chest pain; and breathing difficulties. The gas is sometimes referred to as an insidious agent because its effects may go unnoticed for several hours or days, during which time more serious damage may have occurred. This damage may include pulmonary edema, a condition in which the lungs begin to fill with fluid; cyanosis, a condition in which the lips and mucous membranes turn blue because of lack of oxygen; and a variety of heart problems. Long-term exposure to nitrogen dioxide may result in chronic health problems, such as hemorrhaging (blood loss), lung damage, emphysema, chronic bronchitis, and eventually death.

Words to Know

A molecule that consists of two identical molecules combined with each other.
A compound consisting of very large molecules made of one or two small repeated units called monomers.
A substance that stops the formation of a polymer at some desired point in its production or use.


"Cheminfo: Chemical Profiles Created by CCOHS." (accessed on October 20, 2005).

Holgate, S. T., et al. Air Pollution and Health. New York: Academic Press, 1999.

"NIOSH Pocket Guide to Chemical Hazards: Nitrogen Dioxide." Occupational Health & Safety Administration. (accessed on October 20, 2005).

"Nitrogen Oxides." International Programme on Chemical Safety. (accessed on October 20, 2005).

Patnaik, Pradyot. Handbook of Inorganic Chemicals. New York: McGraw-Hill, 2003, 648-651.

See AlsoNitric Acid