Oxygen
Oxygen is colorless, odorless, gas element denoted by the atomic symbol, O. Its atomic number is 8 and it has an atomic weight of 15.9994. It has a melting point of -360° F (-218° C) and a boiling point of -183 ° F (-297° C).
The discovery of oxygen marked a milestone in the history of science. Until the Renaissance, most European scientists believed in the ancient Greek notion that air was an element, one of four basic substances from which all other things are made. Then, during the 1600s and 1700s, chemists began to identify the different gases contained in air and to clarify the connection between air, combustion, and life.
In the 1770s Swedish chemist Carl Wilhelm Scheele prepared oxygen by heating several different compounds. Scheele called the gas fire air because he found that many substances require it in order to burn. Around the same time that Scheele was conducting his research, English chemist Joseph Priestley obtained oxygen in 1774 while experimenting with mercury and mercuric oxides. Priestley noted various unusual properties of the new gas: a smoldering ember of wood burst into flames when exposed to it, and when a mouse was trapped in a container of the gas, it survived for a longer time than it would have if it had been trapped in a container of ordinary air. Priestley realized that this same gas was produced by plants. In keeping with the scientific theory accepted at that time, he named the gas dephlogisticated air because it readily absorbed phlogiston, the substance scientists thought gave materials their ability to burn. Because his results were published before Scheele's, Priestley often is credited with the discovery of oxygen. Neither Priestley nor Scheele, however, grasped the full implications of their research.
French chemist Antoine-Laurent Lavoisier, who learned of Priestley's work, was the first to explain oxygen's role in the combustion process. When materials burn, they do not acquire phlogiston, Lavoisier discovered, but combine with air, and this increases their weight. Lavoisier realized it was Priestley's dephlogisticated air that was combining with substances during combustion. Believing that acids formed when oxygen compounds were dissolved in water, Lavoisier renamed the gas oxygen, which loosely means acid-producing in Greek.
The first person to liquefy oxygen was French physicist Louis Paul Cailletet (1832-1913). In 1877 he produced small amounts of liquid oxygen by compressing the gas, cooling it as much as possible, and then allowing it to expand.
Since Lavoisier's day, scientists have learned much more about the nature, uses, and abundance of oxygen. Almost half the weight of the Earth's crust can be attributed to compounds which contain oxygen, called oxides. Oxygen makes up about one-fifth of the Earth's atmosphere and also accounts for 85 to 89 percent of the mass of water (H2O). Even half the weight of the human body can be attributed to oxygen. All told, oxygen is one of the most abundant elements on Earth. It is essential for the survival of plant and animal life, and only a few kinds of germs and organisms can survive without free oxygen.
When humans and animals breathe oxygen from the air, it enters the bloodstream in the lungs and is carried by hemoglobin to every cell in the body. Similarly, fish and other aquatic creatures "breathe" dissolved oxygen through their gills. Once it reaches the cells, oxygen engages in a complex series of chemical reactions that provide the organism with energy.
Under ordinary conditions, oxygen is a colorless, odorless, and tasteless gas. It usually forms molecules containing two atoms (O2), but can form a gas called ozone, whose molecules contain three oxygen atoms. The atmosphere's ozone layer helps shield the Earth from the sun's harmful ultraviolet rays, but not all of oxygen's properties are beneficial. Ozone produced at ground level can be a toxic pollutant, and many biochemists think that certain oxidizing agents, which remove electrons from other substances, can gradually damage the body's cells. It is also well known that the rusting of metal is caused by the slow process of oxidation.
Because oxygen promotes combustion so effectively, it is in great demand for industrial purposes. The production of steel consumes the largest amount of commercial oxygen, which is used to burn out impurities in liquid iron. In rocket engines, a mixture of liquid oxygen and fuel provides the propulsive thrust needed to launch rockets. Oxygen is also used in hospitals, where it is piped into patient wards and operating rooms. Breathing pure oxygen relieves respiratory stress in patients with lung or heart problems, and oxygen is often mixed with other gases for anesthesia. Most commercial oxygen is produced by liquefying air, then separating the air's individual components.
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