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Encyclopedia Britannica - Main :: BLA-BOS |
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BORON (symbol B, atomic weight I I) , one of the non-metallic elements, occurring in nature in the form of boracic (boric) acid, and in various borates such as borax, tincal, boronatrocalcite and boracite. It was isolated by J. Gay Lussac and L. Thenard in 18o8 by heating boron trioxide with potassium, in an iron tube. It was also isolated at about the same time by Sir H. Davy, from boracic acid. It may be obtained as a dark brown amorphous powder by placing a mixture of Io parts of the roughly powdered oxide
boron remains as a brown powder, which may by filtered off and dried. H. Moissan (Ann. Chim. Phys., 1895, 6, p. 296) heats three parts of the oxide
ordinary temperatures; and burns on strong ignition with production of the oxide B2O3 and the nitride BN. It combines directly with fluorine at ordinary temperature, and with chlorine, bromine and sulphur
Wohler and H. Sainte-Claire Deville obtained a grey product, from which, on dissolving out the aluminium with sodium hydroxide, they obtained a crystalline product, which they thought to be a modification of boron, but which was shown later to be a mixture of aluminium borides with more or less carbon. Boron dissolves in molten aluminium, and on cooling, transparent, almost colourless crystals are obtained, possessing a lustre, hardness and refractivity near that of the diamond. In 1904 K. A. Ktihne (D.R.P. 147,87.1) described a process in which external heating is not necessary, a mixture of aluminium turnings, sulphur
Boron hydride has probably never been isolated in the pure condition; on heating boron trioxide with magnesium filings, a magnesium boride Mg3B2 is obtained, and if this be decomposed with dilute hydrochloric acid a very evil-smelling gas, consisting of a mixture of hydrogen and boron hydride, is obtained. This mixture burns with a green flame forming boron trioxide; whilst boron is deposited on passing the gas mixture through a hot tube, or on depressing a cold surface in the gas flame. By cooling it with liquid air Sir W. Ramsay and H. S. Hatfield obtained from it a gas of composition B3H3. The mixture probably contained also some BH3 (W. Ramsay and H. S. Hatfield, Proc. Chem. Soc., 17, p. 152). Boron fluoride BF3 was first prepared in 1808 by Gay Lussac and L. Thenard and is best obtained by heating a mixture of the trioxide and fluorspar with concentrated sulphuric acid. It is a colourless pungent gas which is exceedingly soluble in water. It fumes strongly in air, and does not attack glass. It rapidly absorbs the elements of water wherever possible, so that a strip of paper plunged into the gas is rapidly charred. It does not burn, neither does it support combustion. A saturated solution of the gas, in water, is a colourless, oily, strongly fuming liquid which after a time decomposes, with separation of metaboric acid, leaving hydrofluoboric acid HFBF3 in solution. This acid cannot be isolated in the free condition, but many of its salts are known. Boron fluoride also combines with ammonia gas, equal volumes of the two gases giving a white crystal-line solid of composition BF3NH3; with excess of ammonia gas, colourless liquids BF3.2NH3 and BF3.3NH3 are produced, which on heating lose ammonia and are converted into the solid form.Boron chloride BC13 results when amorphous boron is heated in chlorine gas, or more readily, on passing a stream of chlorine over a heated mixture of boron trioxide and charcoal, the volatile product being condensed in a tube surrounded by a freezing mixture. It is a colourless fuming liquid boiling at 1718 C, and is readily decomposed by water with formation of boric and hydrochloric acids. It unites readily with ammonia gas forming a white crystalline solid of composition 2BCI3.3NH3. Boron bromide BBr3 can be formed by direct union of the two elements, but is best obtained by the method used for the preparation of the chloride. It is a colourless fuming liquid boiling at 90.5 C. With water and with ammonia it undergoes the same reactions as the chloride. Boron and iodine do not combine directly, but gaseous hydriodic acid reacts with amorphous boron to form the iodide, BI3, which can also be obtained by passing boron chloride and hydriodic acid through a red-hot porcelain
Boron nitride BN is formed when boron is burned either in air or in nitrogen, but can be obtained more readily by heating to redness in a platinum crucible a mixture of one part of anhydrous borax with two parts of dry ammonium chloride. After fusion, the melt is well washed with dilute hydrochloric acid and then with water, the nitride remaining as a white powder. It can also be prepared by heating borimide B2(NH)3; or by heating boron trioxide with a metallic cyanide. It is insoluble in water and unaffected by most reagents, but when heated in a current of steam or boiled for some time with a caustic alkali, slowly decomposes with evolution of ammonia and the formation of boron trioxide or an alkaline borate; it dissolves slowly in hydrofluoric acid. Borimide B2(NH)3 is obtained on long heating of the compound B2S3.6NH3 in a stream of hydrogen, or ammonia gas at 115120 C. It is a white solid which decomposes on heating into boron nitride and ammonia. Long-continued heating with water also decomposes it slowly. Boron sulphide B2S3 can be obtained by the direct union of the two elements at a white heat or from the tri-iodide and sulphur at 440 C., but is most conveniently prepared by heating a mixture of the trioxide and carbon in a stream of carbon bisulphide vapour. It forms slightly coloured small crystals possessing a strong disagree-able smell, and is rapidly decomposed by water with the formation of boric acid and sulphuretted hydrogen. A pentasulphide B2S5 is prepared, in an impure condition, by heating a solution of sulphur in carbon bisulphide with boron iodide, and forms a white crystalline powder which decomposes under the influence of water into sulphur, sulphuretted hydrogen and boric acid. Boron trioxide B2O3 is the only known oxide of boron; and may be prepared by heating amorphous boron in oxygen, or better, by strongly igniting boric acid. After fusion the mass solidifies to a transparent vitreous solid which dissolves readily in water to form boric acid (q.v.) ; it is exceedingly hygroscopic and even on standing
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