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Element Carbon, C, Non Metal

About Carbon

Carbon is one of the most important elements in respect both of the variety and wide distribution of its compounds and of the importance which these have in nature as well as in the arts. Although oxygen, hydrogen, and nitrogen are never-failing constituents of living or organised structures, still carbon is frequently called the organic element par excellence, because it is on the combining relations exhibited by this element that the diversity of the substances of the organic kingdom most essentially depends.

But the pre-eminent importance of carbon is due not only to its being a constituent of the substances of which the structures of living things are built up, but much more to its being the expression of the supply of energy which is expended in vital action. For a similar reason, carbon is of importance in the arts, for by far the greatest part of the chemical energy which is set in motion for the accomplishment of the most diverse ends is derived from the chemical transformations of carbon.

Elementary carbon occurs in three different forms, which exhibit relationships to one another similar to those found in the case of sulphur or phosphorus. It exists in two crystalline forms and also in an amorphous state. The different varieties of amorphous carbon are usually, but probably incorrectly, classed together as one kind. Indeed, there are important reasons for thinking that there are several kinds of amorphous carbon, each possessing different properties, but none of which are known in the pure state.

That which is called charcoal is amorphous carbon, in a more or less pure state. On heating organic substances, e.g. substances derived from organisms, especially plants, and containing carbon, a residue of this element is generally obtained, whereas the other elements present, especially oxygen and hydrogen, escape in the form of water and as lower carbon compounds of these elements. Moreover, the residue contains any non-volatile substances which may be present, as well as residual quantities of hydrogen and oxygen, which are larger in amount the lower the temperature of carbonisation.

In the charcoal produced, the structure of the material can in some cases, e.g. when obtained from wood, be recognised; wood charcoal exhibits every cell of the wood well preserved. This is due to the. fact that at the temperatures which are reached under these conditions, carbon is an infusible substance. If the original material has also this same property of infusibility, as is the case with the substance forming the cell-walls of wood, the form is well retained on carbonisation. In other cases, where the original material liquefies either before or during carbonisation, e.g. in the case of sugar, the charcoal which is obtained has the appearance of a mass which has been fused; this, however, is due only to the fact that sugar, not carbon, is fusible.

Sugar charcoal is much purer than wood charcoal, because in it the presence can easily be avoided of non-volatile impurities which are present in the case of wood charcoal, and which, on complete combustion, remain behind as a grey powder, the ash.

Soot is a still purer form of carbon. This is obtained by the combustion, in a small supply of air, of volatile compounds of carbon and hydrogen, of which there are a large number. The hydrogen then combines with the oxygen present, and the carbon is deposited and can be collected in the form of a very fine and light powder. Small quantities of hydrogen compounds which it still contains can usually be got rid of by igniting it with exclusion of air.

The properties of this form of carbon are the well-known black colour, a small density, easy combustibility, small conductivity for heat and electricity, and a low degree of hardness.

All these properties, however, cannot be stated in definite numbers, but are found to vary to some extent, and that, indeed, in the following way. The higher the temperature to which the amorphous carbon was exposed, and the longer that temperature was allowed to act on the carbon, the greater are the density, hardness, conductivity for heat and especially for electricity, and the less is its combustibility. At the same time, the deep black colour passes into a grey one with a somewhat metallic lustre.

It has not yet been settled whether the cause of these changes is that the small particles of which the charcoal consists unite together, or " sinter," at the high temperature to larger particles, or that there are different forms of amorphous carbon which occur mixed together in charcoal, the harder, more dense, and better-conducting of which forms are increasingly produced at higher temperatures. The melting point of charcoal is certainly as high as 3000° or 3500°, the temperature of the electric arc, but it is quite possible that the general property of amorphous substances, of having no definite melting point, is present also in this case, and that, therefore, even at much lower temperatures, an incipient softening may occur which would lead to the formation of larger grains by the caking together of the smaller. In this way, the above-mentioned changes can be partially explained. It appears, however, especially in view of the increase of the hardness and conductivity, to be more appropriate to assume the existence of several forms of amorphous carbon, which differ from one another in the way described, and which in varying proportions make up ordinary charcoal.

Carbon retains the solid state with especial obstinacy. Only at the temperature of the electric arc, about 3500°, does softening and volatilisation occur. Further, there is scarcely a solvent which dissolves carbon to any great extent. The only better-known one is liquid iron, in which carbon dissolves to the extent of a few per cent at comparatively high temperatures, and from which it separates out when the metal solidifies. Under these conditions, however, carbon does not appear in amorphous form, but in the crystalline form of graphite, which will be described later.

When heated in the air, carbon unites with oxygen, and is converted into carbon dioxide.

The fossil charcoal occurring in nature, such as anthracite, coal, and brown coal, consists, it is true, chiefly of carbon, but it also contains hydrogen and oxygen along with small quantities of nitrogen, sulphur, and very varying amounts of ash, i.e. mineral admixtures of all kinds. The different sorts have all been formed in a similar way to wood charcoal, viz. from the remains of previous vegetation by the gradual loss of the other elements and the formation of a residue of carbon. This process has, however, taken place at a low temperature and required very long periods of time. This process of carbonisation has progressed furthest in the case of anthracite, which contains only quite small quantities of hydrogen; not so far in the case of ordinary coal, and least of all in the case of brown coal. The latter substances cannot be regarded as carbon in the strict sense; on the contrary, they consist of derivatives, of complex composition and certainly very rich in carbon, of the substances of which the original plant-structures were built up, or of mixtures of such substances with amorphous carbon.

On heating ordinary coal with exclusion of air, the hydrogen is removed in the form of carbon compounds. This process is carried out on a large scale for two purposes. On the one hand, coal rich in hydrogen is subjected to heating or "dry distillation," and the gases containing carbon which are produced are collected in order to be used, after purification, for illuminating or heating purposes. This manufacture of coal gas plays a very important part, since gaseous fuel possesses important advantages over the solid or liquid. We shall enter into this more fully later.

On the other hand, coal which is poor in hydrogen is also subjected to dry distillation in order to obtain in the residues carbon which is almost free from hydrogen, and which in many cases, especially for metallurgical purposes, is to be preferred to coal containing hydrogen. These coal residues are called coke, and are made on a very large scale.

A point which is of essential importance here is that the greater portion of the sulphur present is removed in the carbonisation, so that in this respect also a purification is effected.

Carbon History

Carbon was discovered in prehistory and was known to the ancients, who manufactured it by burning organic material in insufficient oxygen (making charcoal). The elementary nature of carbon had been discovered by Antoine-Laurent de Lavoisier in late 1780-s. The three well-known allotropes of carbon from ancient times are amorphous carbon, graphite, and diamond. From the antiquity graphite, named by Abraham Gottlob Werner in 1789, was known for its use in drawing/writing. However carbon's history is very complex. It was often confused with other substances with similar physical properties, such as molybdenite (molybdenum sulphide) considered as graphite at one time it is also known as black-lead, plumbago, mineral carbon, and mineral black. In 1779 Karl Scheele specified that graphite may be oxidized extracting carbon dioxide. The international name is originated from "carbo" - coal associated with the ancient root "kar", which means heat. The same root is in the Latin word "cremare" which means "to burn".

Carbon Occurrence

The abundance of carbon in the Earth crust is 0.1% of its mass. As the free element it forms allotropes from differing kinds of carbon-carbon bonds, such as in graphite and diamond. Carbon is a major component of very large masses carbonate rock (limestone, dolomite, marble etc.) Coal is the main source of carbon in mineral form, containing up to 95% of carbon in anthracite (94-97% C), and brown coal (64-80% C), bituminous coal (76-95% C), oil shale (56-78% C), petroleum (82-87% C), combustion and natural gases (up to 99% of methane), turf (53-56% C) as well as bitumen etc. As a carbon dioxide carbon is present in the Earth's atmosphere and hydrosphere (approximately 18%). Living organisms consist of 18% carbon. The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the biosphere, geosphere, hydrosphere, and atmosphere of the Earth through combustion of organic material which oxidizes the carbon it contains, producing carbon dioxide, volcanic eruptions, at the surface of the oceans and so on. Carbon is taken from the troposphere by plants which perform photosynthesis process. Then carbon is released back into the geosphere in many different ways, such as through the decay of animal and plant matter in the soil and, being converted into carbon dioxide if oxygen is present, as CO2 - in atmosphere.

Carbon in vaporized state and in compounds with nitrogen and hydrogen is abundant in the Sun, stars, comets, and in the atmospheres of most planets. It has been found in some stony and iron meteorites.

Carbon has the ability to form long, indefinite chains with interconnecting covalent bonds, primarily in hydrocarbons, which are strong and stable. This property allows carbon to form an infinite number of compounds; in fact, there are more known carbon-containing compounds than all the compounds of the other chemical elements combined except those of hydrogen (because almost all carbon compounds contain hydrogen).


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