Oxygen
| I | INTRODUCTION |
Oxygen, symbol O, colorless, odorless, tasteless,
slightly magnetic gaseous element. In Earth’s crust, oxygen is more abundant
than any other element. Oxygen was discovered in 1774 by the British chemist
Joseph Priestley and, independently, by the Swedish chemist Carl Wilhelm
Scheele; it was shown to be an elemental gas by the French chemist Antoine
Laurent Lavoisier in his classic experiments on combustion. The element’s name
was created by combining the Greek words oxys “acid” and genes
”forming,” because oxygen is a common component of acids.
| II | PROPERTIES AND OCCURRENCE |
Gaseous oxygen can be condensed to a pale blue
liquid that is strongly magnetic. Pale blue solid oxygen is produced by
compressing the liquid. The atomic weight of oxygen is 15.9994; at atmospheric
pressure, the element boils at -182.96°C (-297.33°F), melts at -218.4°C
(-361.1°F), and has a density of 1.429 g/liter at 0°C (32°F).
Oxygen composes 21 percent by volume or 23.15
percent by weight of the atmosphere; 85.8 percent by weight of the oceans (88.8
percent of pure water is oxygen); and, as a constituent of most rocks and
minerals, 46.7 percent by weight of the solid crust of the Earth. Oxygen
comprises 60 percent of the human body. It is a constituent of all living
tissues; almost all plants and animals, including all humans, require oxygen, in
the free or combined state, to maintain life. See Respiration.
Three structural forms of oxygen are known:
ordinary oxygen, containing two atoms per molecule, formula O2;
ozone, containing three atoms per molecule, formula O3; and a pale
blue, nonmagnetic form, O4, containing four atoms per molecule, which
readily breaks down into ordinary oxygen. Three stable isotopes of oxygen are
known; oxygen-16 (atomic mass 16) is the most abundant. It comprises 99.76
percent of ordinary oxygen and was used in determination of atomic weights until
the 1960s (see Atom).
Oxygen is prepared in the laboratory from
salts such as potassium chlorate, barium peroxide, and sodium peroxide. The most
important industrial methods for the preparation of oxygen are the electrolysis
of water and the fractional distillation of liquid air. In the latter method air
is liquefied, then slowly warmed and allowed to evaporate. Nitrogen, argon, and
other trace gases in the liquid air are more volatile and boil off first,
leaving the oxygen. The oxygen is then evaporated and collected, leaving behind
other trace gases with even higher boiling temperatures.
Oxygen is a component of many organic and
inorganic compounds. It forms compounds called oxides with almost all the
elements, including some of the noble gases. A chemical reaction in which an
oxide forms is called oxidation. The rate of the reaction varies with
different elements. Ordinary combustion, or burning, is a very rapid form of
oxidation. In spontaneous combustion, the heat evolved by the oxidation reaction
is sufficiently great to raise the temperature of the substance to the point
that flames result. For example, phosphorus combines so vigorously with oxygen
that the heat liberated in the reaction causes the phosphorus to melt and burn.
Certain very finely divided powders present so much surface area to the air that
they burst into flame by spontaneous combustion; they are called pyrophoric
substances. Sulfur, hydrogen, sodium, and magnesium combine with oxygen less
energetically and burn only after ignition. Some elements, such as copper and
mercury, form oxides slowly, even when heated. Inactive metals, such as
platinum, iridium, and gold, form oxides only through indirect methods. For
discussion of oxides of elements see separate articles on each element.
| III | USES |
Large amounts of oxygen are used in
high-temperature welding torches, in which a mixture of oxygen and another gas
produces a flame of much higher temperature than is obtained by burning gases in
air. Oxygen is administered to patients whose breathing is impaired and also to
people in aircraft flying at high altitudes, where the poor oxygen concentration
cannot support normal respiration. Oxygen-enriched air is used in open-hearth
furnaces for steel manufacture.
Most of the oxygen produced in the United
States is used to make a mixture of carbon monoxide and hydrogen called
synthesis gas, used for the synthesis of methanol and ammonia.
High-purity oxygen is used also in the metal-fabrication industries; in liquid
form it is of great importance as a propellant for guided missiles and rockets
(see Rocket: Liquid Propellants).
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