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Physical properties of Boron






Boron is a black solid of density 2.34 and compressibility 0.3×10-6 per atmosphere at 20°. In hardness it is inferior only to diamond. Its fracture is conchoidal; it shows no signs of microcrystalline structure. It is inferior to diamond in its toughness or strength. Under atmospheric pressure boron melts at about 2200°. It passes into vapour at temperatures considerably below the melting-point, the vapour tension becoming noticeable at 1600°. The electrical conductivity of a piece of massive boron at ordinary temperatures is extremely small, but between room temperature and that corresponding to a dull red heat it increases to about two million times its initial value. For instance, a piece of boron which had a resistance of 5.62×106 ohms at 27° had a resistance of only 4.60×104 ohms at 180° C. At a dull red heat the resistance had fallen to 5 ohms. Over a short temperature interval the resistance of boron is an exponential function of the temperature. At 23° the specific resistance is about 1.7×106 ohms per cm. cube; at 0° the value is about 2×106. The specific resistance of boron is enormously decreased at ordinary temperatures by the introduction into it of small amounts of other elements, and at the same time the very large negative temperature-coefficient is diminished. Thus, a few tenths per cent, of carbon introduced into boron reduces the resistance to about one-twelfth its value at ordinary temperatures.

The atomic refraction of boron in saturated compounds for the D line is 5.3 or 3.2, and for the Hα line 5.4 or 3.1, according as the Gladstone and Dale or the Lorenz and Lorentz formula is employed.

Spectrum boric acid
Band spectrum of Boric Acid.
The spark and arc spectra of boron consist simply of three lines in the ultraviolet, 3451.50, 2497.83, and 2496.89, the pair asterisked being more persistent than the other line. Two band spectra, however, are associated with boron. The green flames due to boric acid, alkyl borates, and boron fluoride all have practically identical band spectra; the bands are headless, and the most prominent are in the green region (see fig.). They have been observed and measured by numerous scientists. The same bands are observed in the arc spectra of boron and boron sesqui-oxide, and in the spark spectrum of a solution of boric acid in hydrochloric acid, and they are attributed to oxide of boron; they do not occur in the spark spectrum of boron itself. Another series of bands, which have definite heads and degrade towards the red, is found in the spectrum of boron trichloride or methyl borate in the afterglow of active nitrogen; these bands also occur to some extent in the arc spectrum of boron and its oxide, and are attributed to boron nitride but they do not occur in the spark spectrum of boron in nitrogen.

Pure boron may be strongly heated in air without undergoing any perceptible oxidation. It is oxidised to boric acid when heated with concentrated nitric acid, but the rate of oxidation is very slow. It does not combine with either copper (Weintraub) or magnesium (Ray) at a red heat. Its other chemical properties are unknown.

Moissan's " amorphous boron " is a maroon-coloured powder of specific gravity 2.45. Its specific heat increases rapidly with rise of temperature, the mean value being 0.3066 between 0° and 100°, 0.3407 between 0° and 192°, and 0.3573 between 0° and 235°.

The chemical properties of "amorphous boron" have been described in detail by Moissan, and are given in the following paragraphs. It must, however, be remembered that these properties refer to a substance that has been shown by Weintraub to contain 4 to 5 per cent, of oxygen. It will be noticed that several of the properties mentioned are not the properties of pure boron.

Boron unites directly with fluorine at the ordinary temperature, with chlorine at 410°, and with bromine at 700°, but is without action on iodine. With the halogen acids it reacts with greater difficulty; hydrogen fluoride attacks it at a dull red heat, hydrogen chloride at a bright red heat, the products being hydrogen and boron fluoride or chloride. Hydrogen iodide has no action on boron. When heated in oxygen, boron ignites with the production of heat and light, and it burns in air at 700°. It unites readily with sulphur at 600°, producing the sesqui-sulphide, and combines with selenium at a higher temperature; it does not combine directly with tellurium. The direct combination of boron and nitrogen occurs very slowly at 900°, but rapidly at 1250°. Neither phosphorus nor arsenic vapour at 750° will combine with boron, and antimony may be fused with it without the production of any chemical change. When heated with carbon in the electric arc in an atmosphere of hydrogen, crystalline boron carbide is produced.

Boron does not unite directly with the alkali metals. On the other hand, it unites with magnesium at a dull red heat, and with iron and aluminium at a higher temperature. It also unites with silver and platinum.

Boron acts as a powerful reducing agent. It decomposes steam at a red heat, reduces iodic acid on warming, with the liberation of iodine, and at a temperature just below a dull red heat it reduces sulphur dioxide. At a dull red heat the oxides of arsenic and carbon dioxide are reduced by boron, at 800° phosphoric anhydride is reduced, and at 1200° the reduction of carbon monoxide and silica can be effected. At a bright red heat boron burns in either nitrous or nitric oxide, producing boron oxide and nitride. Concentrated sulphuric and nitric acids are readily reduced by boron, which becomes oxidised to boric acid.

A large number of oxides and salts are reduced by boron. The oxides of copper, tin, lead, antimony, and bismuth are reduced when gently warmed with boron, the mixtures becoming incandescent. At a red heat the oxides of iron and cobalt are reduced, but not the alkaline earth oxides. Silver fluoride is violently reduced by boron at ordinary temperatures, the fluorides of zinc and lead at a red heat, but not the alkali and alkaline-earth fluorides. Chlorides are less easy to reduce, but mercuric chloride can be reduced to mercury at 700°. When boron is fused with an alkali hydroxide, a violent evolution of hydrogen occurs, and an alkali borate is produced.

Reductions may be effected at the ordinary temperature with certain aqueous solutions. Thus, potassium permanganate is slowly reduced, and precipitates of the respective metals are produced by the action of boron on aqueous solutions of silver nitrate and the chlorides of gold, platinum, and palladium.

Boron will probably receive various practical applications in the near future, owing to its remarkable electrical properties. Its volatility prevents its employment in the manufacture of electric lamp filaments. Boron is of great use in the purification of copper; if 0.03 to 0.1 per cent, be added to molten copper before it is cast, the metal is deoxidised and purified to a remarkable extent. It is cheaper, however, to use "boron suboxide" for the purpose. A little boron increases the breaking stress of steel.


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