Carbon and its Compounds

Coal

Coal is formed in nature by the 'carbonisation' of wood. Conversion of wood to coal under the influence of high temperature, high pressure, and in the absence of air is termed carbonisation.

Amongst coal varieties, anthracite is the purest form. It contains about 94 - 95% of carbon. The common variety of coal is bituminous coal; it is black, hard and burns with smoky flame.

Wood Charcoal

When wood is heated strongly in a very limited supply of air, wood charcoal is obtained. This process is called destructive distillation of wood. The volatile products are allowed to escape. Charcoal is a black, porous and brittle solid. It is a good adsorbent. Charcoal powder adsorbs colouring matter from solutions, and poisonous gases from the air. Charcoal is also a good reducing agent.

Animal Charcoal

Animal charcoal (or Bone charcoal) is obtained by destructive distillation of bones. It contains about 10-12% of amorphous carbon.

Sugar Charcoal

It is obtained by heating sugar in the absence of air. Sugar charcoal is the purest form of amorphous carbon.

Sugar charcoal becomes activated charcoal when it is powdered to particle size of about 5 µ and heated at about 1000 K in vacuum. Activated charcoal has an increased adsorption capacity.

Lamp Black

Lamp black is manufactured when tar and vegetable oils (rich in carbon) are burnt in an insufficient supply of air and the resulting soot is deposited on wet blankets hung in a room. Lamp black is a velvety black powder. It is used in the manufacture of India ink, printer's ink, black paint and varnishes and carbon papers.

Carbon Black

When natural gas is burned in limited supply of air, the resulting soot is deposited on the underside of a revolving disc. This is carbon black and it is then scraped off and filled in bags. It differs from lamp black in being not so greasy. Carbon black is added to the rubber mix used for making automobile tyres, and has replaced the use of lampblack for a number of purposes.

Gas Carbon and Petroleum coke

Carbon scraped that is from the walls of the retort used for the destructive distillation of coal in called gas carbon. During refining of crude petroleum, petroleum coke is deposited on the walls of the distillation tower.

Both, gas carbon and petroleum coke are used for making electrodes in dry cells and are good conductors of electricity.

Fullerenes

Fullerenes are allotropes of carbon that were discovered as recently as 1985. They have been found to exist in the interstellar dust as well as in geological formations on earth. They are large cage like spherical molecules with formulae C₃₂, C₅₀ C₆₀, C₇₀, C₇₆, C₈₄ etc. The most commonly known fullerene is C₆₀ which is named as 'buckminster fullerene after the designer of the geodesic dome, American architect Buckminister.

C₆₀ molecule has marvelously symmetrical structure. It is a fused-ring of aromatic system containing 20 hexagons and 12 pentagons of C atoms. The structure bends around and closes to form a soccer ball shaped molecule. It is therefore, called buckyball also. Fullerene looks different from diamond and graphite. It is a yellow powdery substance, which turns pink on dissolution in solvents like toluene. It polymerizes on exposure to U.V. radiations.

Fullerenes are fascinating because they show unusual characteristics and applications like:

• They are wonderful lubricants because the balls can roll between the surfaces.
• Alkali compounds of C₆₀ (A₃C₆₀) are super conducting materials even at high temperatures of the order of 10-40 K.