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ozone(ō`zōn), an allotropic form of the chemical element oxygenoxygen,
gaseous chemical element; symbol O; at. no. 8; interval in which at. wt. ranges 15.99903–15.99977; m.p. −218.4°C;; b.p. −182.962°C;; density 1.429 grams per liter at STP; valence −2.
..... Click the link for more information. (see allotropyallotropy
[Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes.
..... Click the link for more information. ). Pure ozone is an unstable, faintly bluish gas with a characteristic fresh, penetrating odor. The gas has a density of 2.144 grams per liter at STPSTP
or standard temperature and pressure,
standard conditions for measurement of the properties of matter. The standard temperature is the freezing point of pure water, 0°C; or 273.15°K;.
..... Click the link for more information. . Below its boiling point (−112°C;) ozone is a dark blue liquid; below its melting point (−193°C;) it is a blue-black crystalline solid. Ozone is triatomic oxygen, O3, and has a molecular weight of 47.9982 atomic mass units (amu). It is the most chemically active form of oxygen. It is formed in the ozone layerozone layer
region of the stratosphere containing relatively high concentrations of ozone, located at altitudes of 12–30 mi (19–48 km) above the earth's surface.
..... Click the link for more information. of the stratosphere by the action of solar ultraviolet light on oxygen. Although it is present in this layer only to an extent of about 10 parts per million, ozone is important because its formation prevents most ultraviolet and other high-energy radiation, which is harmful to life, from penetrating to the earth's surface. Ultraviolet light is absorbed when its strikes an ozone molecule; the molecule is split into atomic and diatomic oxygen: 03+ ultraviolet light →0+02. Later, in the presence of a catalyst, the atomic and diatomic oxygen reunite to form ozone. Some environmental scientists fear that certain human-produced pollutants (e.g., nitric oxide, NO) may interfere with this delicate balance of reactions that maintains the ozone's concentration, possibly leading to a drastic depletion of stratospheric ozone. Ozone is also formed when an electric discharge passes through air; for example, it is formed by lightning and by some electric motors and generators. Ozone is produced commercially by passing dry air between two concentric-tube or plate electrodes connected to an alternating high voltage; this is called the silent electric discharge method. Ozone is used commercially as a disinfectant and decontaminant for air and water, and as a bleaching agent for waxes, oils, and other organic compounds. The major commercial use is in the production by ozonolysis of azelaic acid (used in making plastics); it is also used in the synthesis of cortisone and certain synthetic sex hormones. Ozonization, the reaction of ozone with the double or triple bonds of unsaturated organic molecules, is useful in determining the structure of organic compounds.
an allotropic form of oxygen. Melting point, — 192.7°C; boiling point, — 112°C. Ozone is an explosive blue gas with a characteristic pungent odor. Unlike the diatomic molecule of ordinary oxygen (O2), the ozone molecule is triatomic (O3). Ozone was first discovered in 1785 by the Dutch physicist M. van Marum, who determined that air acquires characteristic oxidizing properties and a fresh odor after electric sparks are passed through the medium. One liter of ozone weighs 2.1445 g under normal conditions. Gaseous ozone is 1.5 times as dense as oxygen and 1.62 times as dense as air. At temperatures below the boiling point, ozone converts to a dark blue liquid with a density of 1.71 g/cm3 at — 183°C. It is diamagnetic in the gaseous state and slightly paramagnetic in liquid form. Ozone has a water solubility of 0.394 g/l at 0°C, which is 15 times higher than the solubility of oxygen. Ozone forms in the reversible reaction 3O2 + 68 kilocalories (285 kilojoules) ⇆ 2O3. The ozone molecule, O3, is unstable and undergoes spontaneous conversion into O2 with the liberation of heat energy. Small concentrations of pure ozone decompose gradually; the decomposition of large concentrations is marked by an explosion. The heating and subsequent interaction of ozone with small quantities of organic substances, certain metals, or metal oxides significantly accelerate the conversion process. Inversely, the presence of small quantities of HNO3 stabilizes ozone. Storage in containers made of glass, certain plastics, or pure metals at a constant temperature of —78°C also nearly prevents ozone decomposition.
Ozone is one of the strongest oxidizing agents—considerably stronger than O2. It is capable of oxidizing most elements, including all metals except gold and the platinum metals. Ozonides form upon interaction of ozone with certain inorganic and organic compounds. The presence of ozone in a gas mixture can be determined by the reaction
O3 + 2KI + H2O = I2 + O2 + 2KOH
O2 does not react with KI.
Ozone forms in processes that are accompanied by the liberation of atomic oxygen, for example, the decomposition of peroxides and phosphorus oxidation. It is commercially prepared in ozonizers by passing air or oxygen through a silent electrical discharge at low temperatures. Ozone and diatomic oxygen are easily distinguished, since O3 liquefies more readily than O2.
Because of its strong oxidizing properties, ozone is used to synthesize many organic substances and to bleach paper and oils. Its destructive effect on microorganisms makes it a highly suitable disinfectant for water and air; the disinfection process is called ozonization. Since ozone is extremely toxic (more so than carbon monoxide), the maximum permissible concentration of O3 in the air is 10-5 percent.