OXYGEN 专业英语ppt

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13、Oxygen

The air we breathe is a mixture of several gases including nitrogen,oxygen,carbon dioxide,argon,water vapor and in urban environments Sulphur dioxide. The two main gases nitrogen and oxygen, occupy between them about 99 percent of any given volume of air, the remaining constituents being present only in very small proportions. Oxygen occupies about 21 percent by volume and 23 percent by weight of the atmosphere and is the commonest of all elements in the Earth’s crust as a whole masking up almost half of it in one form or another.（地球表面上有将近一半被以一种形式或另一种形式存在的氧覆盖着）

Oxygen was discovered as a separate element, but not named as such, by two scientists working independently in 1774, Joseph Priestley in England and C. W. Scheele in Sweden who was investigating what happens during heating or burning of materials. The modern theory of burning is attributed to Antoine Lavoisier, a Frenchman, who, after meeting Priestley in Paris in 1774, confirmed that oxygen is the part of air which combines with burning materials to cause an increase in weight. It was Lavoisier who gave it the name oxygen.

Before 1774, scientists had been at odds over what actually happens what something is burnt. Some materials, such as paper, wood and coal, appear to lose weight when they burn and others, such as copper magnesium and iron filings, gain weight after heating. One explanation was based on the supposition that there existed in all matter an imaginary material called ‘phlogiston’ that could be burnt to a greater or lesser extent. On burning, the phlogiston escaped and the substance lost weight. The idea of phlogiston did not, however, explain an increase in weight and to get round this it was even assumed that sometimes phlogiston could have negative weight! We now know that in fact all material when they are heated or burned in air absorb oxygen and gain in weight. The apparent loss of weight when wood coal or other materials are burnt is because of the smoke given off and lost. Controlled heating where no smoke is lost has shown conclusively that these materials too absorb oxygen from the air. All materials require the presence of oxygen to a greater or lesser degree for burning to take place. (The observation that gunpowder will burn apparently without oxygen is explained by the fact that there is a certain amount of oxygen present in saltpeter, which is one of the constituents of gunpowder.) One of the tests of oxygen is its ability to relight a glowing splint, so if relighting occurs on insertion of a splint into a gas we know oxygen is present.

Oxygen is a most important element. It is colorless, odorless and tasteless, slightly soluble in water(which is essential for water life), can be liquefied at -183℃ and has approximately the same density as air. The industrial uses of oxygen are in chemical processing and, together with acetylene, in oxy-acetylene cutting and welding of metals. Medical use is made of oxygen to assist breathing in high altitude flying and climbing, submarines and mines.

When a material is heated in air it absorbs oxygen and a chemical reaction takes place resulting in the formation of the oxide of the material. Some examples are copper to form copper oxide, magnesium to form magnesium oxide, carbon to form carbon dioxide (in this case each molecule of carbon dioxide contains two atoms of oxygen, hence the prefix‘di-’in dioxide), and phosphorus to form phosphoric oxide, and so on.

Oxide formation, especially on metals, may or may not be advantageous. In some cases the oxide once formed protects the metal underneath, but in others, where the metal is being used as a conductor of electricity, for example, and oxide of the metal is a non-conductor, had electrical connections may ensue. Another everyday example of oxide formation being distinctly


disadvantageous is the process of rusting. We are all familiar with the changes that take place on the surface of iron or iron-based metals which are left out of doors unprotected by paint or other material. The reddish-brown flakes of rust which form are in fact hydrated iron oxide. Rusting is a similar process to burning in that the iron absorbs oxygen from the atmosphere. However, in this case heat is not required but water must be present as well as traces of carbon dioxide. Rusting is quite a complicated process, in which ferrous carbonate is first formed, then oxidized to ferric carbonate, which absorbs water to give hydrated ferric oxide (rust) and some carbon dioxide. Rust is a major problem in industry and also in domestic surroundings since much of our construction work in machines and buildings uses iron or iron based materials. Left to itself the rusting process continues until all or most of the iron in the vicinity is consumed and holes or major areas of weakened metal are left behind. Any suitable material to prevent the metal remaining in close contact with air and water may be used as a rust preventative.

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