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Carbon in Nature
Carbon is an extremely important element and was known from the earliest times in the form of charcoal, soot, etc. Although the amount of carbon in the earth's crust is only eleventh by mass, it ranks as high as oxygen and hydrogen, in importance. In nature, it occurs mainly as coal and petroleum and in the form of calcium carbonate or magnesium carbonate rocks (calcite, limestone, dolomite, marble and chalk). It is also present in almost all the foods we eat. Thus, millions of carbon compounds exist in nature and many thousands are synthetically prepared.
Allotropy and Allotropes of Carbon
The existence of an element in more than one form, in the same physical state, is called allotropy. The various forms are called allotropes. Allotropy is the result of different arrangement of the atoms in the crystal structure of the element. As a result, the allotropes exhibit very different physical properties. Thus diamond and graphite are the allotropes of carbon. Many other elements like sulphur, phosphorus, etc., also exhibit this allotropic property.
Diamond
Diamond is the purest form of natural carbon. It occurs as small crystals embedded in rocks. These are supposed to have been formed by the crystallization of carbon under extreme pressure and temperature in the interior of the earth. Nowadays, synthetic industrial diamonds are being manufactured by subjecting graphite to very high temperatures and pressures. Carbon atoms in diamond have tetrahedral structure. Each atom of carbon is surrounded by four other atoms that together forms the tetrahedral structure.
Graphite
Unlike the tetrahedral arrangement of atoms in diamond, the carbon atoms in graphite are arranged in the form of hexagonal rings in layers (Fig.10.2). Each carbon is bonded to only three other carbon atoms in that layer. Different layers of graphite are held together by rather weak forces. Hence they can slide over one another. This is one reason why graphite scales off easily and can mark impressions on substrates. Because of this property, it is also used as a lubricant.
Amorphous Carbon - Coke
Apart from diamond and graphite, which are crystalline forms of carbon, all other forms of carbon are amorphous allotropes of carbon.
Coke Coke is the amorphous allotrope of carbon, which is derived from coal. Coal is not a true allotrope as elements like sulphur are contained in it, in the combined form. When coal undergoes destructive distillation, it yields two allotropes of carbon, namely coke and gas carbon. Destructive distillation is a chemical process, which involves is the breaking up of a complex substance by heating it in the absence of air. To understand this process, the following experiment is performed.
Amorphous Carbon - Lamp Black
Lamp Black is also known as Soot. Soot is obtained by the incomplete combustion of carbonaceous, fuels, especially oil fuels, in limited supply of air. The soot settles on the cooler parts of the chamber, and can be collected by scrapping it. It is used in the manufacture of printing ink, shoe polish, carbon paper, typewriter ribbons, black paints, varnishes etc.
Amorphous Carbon - Wood Charcoal
Wood charcoal is obtained by the destructive distillation of wood. The chief products formed are wood charcoal, wood tar, pyroligneous acid and wood gas.
Amorphous Carbon - Sugar Charcoal
Sugar charcoal can be obtained by dehydrating cane sugar, either by treating it with concentrated sulphuric acid or by heating it in the absence of air. This process can easily be demonstrated by the following experiment.
Animal Charcoal
Bone Charcoal It is prepared by the destructive distillation of bones of animals. Bones obtained from slaughter houses, are first treated with certain bacteria to remove all the fleshy remnants. Then the bones are taken in a furnace and heated strongly in the absence of air. The solid matter obtained contains nearly 10% of carbon, and almost 90% of calcium phosphate. It is then treated with dilute hydrochloric acid, which dissolves the calcium phosphate, leaving behind bone charcoal. This can be separated by filtration.
Summary
Carbon is the eleventh most abundant element in the earth's crust by mass.
Carbon occurs both in free state as diamond, coal etc. and also in the combined form as CO2.
