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107

Stories About Chemistry

INDEX

53. A Luminous Jet

How many states of matter are there? Modern physicists have counted up no less than seven. Three of them are widely known: gas, liquid, and solid. Strictly speaking, we practically never encounter any others in our daily life. Chemistry has also contented itself with these three for centuries. And only during the last decade has it begun to take an interest in the fourth state of matter, plasma.

Plasma is also a gas, if you like, but not an ordinary one. Besides neutral atoms and molecules it contains ions and electrons. An ordinary gas also contains ionized particles, and the higher its temperature the more of them it contains. Therefore there is no distinct boundary line between an ionized gas and plasma. But it is conventionally considered that a gas has turned into plasma when it begins to display the principal properties of the latter, say, high electrical conductivity.

Paradoxical as it may seem at first glance, plasma is the master in the universe. The matter of the Sun and the stars, as well as the gases of outer space are in the plasma state. All this is natural plasma. On Earth it has to be prepared artificially, in special apparatuses called plasmotrons. In them various gases (helium, hydrogen, nitrogen, argon) are converted to plasma by means of an electric arc. The luminous plasma jet is compressed by the narrow channel of the plasmotron nozzle and by a magnetic field, so that a temperature of several tens of thousands of degrees develops in it.

Chemists had long dreamt of such temperatures, because the role of high temperatures for many chemical processes can hardly be overestimated. Now this dream has come true: a new branch of chemistry known as plasmochemistry, or the chemistry of “cold” plasma, has been born.

Why “cold” plasma? Because there is also “hot” plasma with a temperature of up to a million degrees. This is the plasma with which physicists are trying to achieve thermonuclear synthesis, i.e., to accomplish the controlled nuclear reaction of transformation of hydrogen into helium.

But chemists are quite content with “cold” plasma. To investigate the course of chemical processes at a temperature of ten thousand degrees - what could be more alluring?

Sceptics thought this work would be in vain, because in such a hot atmosphere all substances without exception would share the same fate: they would all be destroyed, and even the most complex molecules would be dissociated into separate atoms and ions.

Actuality is far more complex. Plasma not only destroys, but creates too. New chemical compounds can readily be synthesized in it, some of which cannot be obtained by other means.

These are strange substances never described in any chemical textbook: Al2O, Ba2O3, SO, SiO, CaCl, etc. In them the elements display unusual, anomalous valences. This is all very interesting, but plasmochemistry has set itself more important tasks, namely, the cheap and rapid production of already known valuable substances.

And now a few words about its achievements.

Acetylene is a very important starting material for many organic syntheses, e. g., for the production of plastics, rubbers, dyes, and medicinals. But acetylene is still prepared as of old, by decomposing calcium carbide with water, which is expensive and inconvenient.

In the plasmotron everything is different. Plasma made from hydrogen has a temperature of 5000 degrees. The hydrogen plasma jet carries its enormous energy into a special reactor to which methane is fed. The methane is mixed vigorously with the hydrogen and in the course of one ten-thousandth of a second more than 75 per cent of the methane changes into acetylene.

Isn’t that ideal? We should say so! But alas, there is always a hitch somewhere. If we leave the acetylene for an extra instant in the high temperature zone of the plasma it begins to decompose. Hence the temperature must be lowered swiftly to a safe level. There are different ways of accomplishing this, but it is the main technical difficulty. So far only 15 per cent of the acetylene formed can be saved from dissociation. But even that is not so bad!

A method of decomposing cheap liquid hydrocarbons plasmochemically to form acetylene, ethylene, and propylene has been developed in the laboratory.

A very important problem that has still to be coped with is the fixation of atmospheric nitrogen. The chemical production of nitrogen-containing compounds, e.g. ammonia, is a very laborious, involved and expensive operation. A few decades ago attempts were made to synthesize nitrogen oxides electrically on an industrial scale, but the economics of the process was too low. Here also plasmochemistry holds much more promise.


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