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Chemical Information on Diamonds

Diamond is the purest crystalline allotropic form of carbon, occurring in nature in the form of rounded pebbles in South Africa, Russia, Australia and Brazil. Large deposits have been found in Congo and South Africa. In nature, diamond occurs as octahedral crystals. The largest piece of diamond (weighing approximately 1.3 lb.) known as Cullinan Diamond, was found in an South African mine at Kimberley in 1905.

  1. Preparation

    • Artificial diamonds were prepared by heating pure sugar charcoal and iron in a graphite crucible at about 3,000 degrees C in an electric furnace. At this high temperature, carbon was dissolved in iron. Now, when the molten mass was cooled rapidly by plunging it into a bath of molten lead and then treated with concentrated hydrochloric acid, crystals of artificial diamond were obtained.

    Structure

    • X-ray analysis has shown that the crystal of diamond is made up of a number of tetrahedral units. Each tetrahedral unit contains carbon atoms at its center and this carbon atom is linked with other four carbon atoms placed at the four corners of the tetrahedron. Carbon atoms of each tetrahedron are also linked by strong C-C covalent bonds with carbon atoms of each of the neighboring tetrahedrons. Since diamond consists of a number of tetrahedral units, it is a giant molecule.

    Physical Properties

    • Diamond crystallizes in the cubic system. Good varieties are all transparent, colorless or colored. Due to the giant molecular nature of the structure of diamond, it is a very hard substance and at the same time very brittle. In fact, diamond is the hardest natural substance known so far. Due to its great hardness, diamond is used for making glass cutters and rock borers. The melting point of diamond is very high, because a large amount of head energy is needed in order to break strong C-C covalent bonds present in diamond. Its relative density is also very high.

    Chemical Properties

    • Diamond is not affected by almost all chemical reagents, so it is insoluble in all solvents. Diamond burns in air to form carbon dioxide at 900 degrees C. With fluorine at 700 degrees C, diamond gives carbon tetrafluoride. It is slowly oxidized by a warm mixture of potassium dichromate and concentrated hydrogen sulfide at 200 degrees C to form carbon dioxide. Diamond is also slowly acted on by fused sodium carbonate. It is stable in vacuum up to 1,500 degrees C, but when heated to 1,800 to 2,000 degrees C, diamond is converted into graphite.

    Uses

    • The colorless diamonds are used as precious jewelry stones because of their ability to reflect and refract light. Diamonds are also used for drawing of thin wires.

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