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Physical Properties of Tungsten

Metallic tungsten, according to its method of preparation, is silver white, resembling highly polished platinum, or is crystalline in structure, or is a grey powder. When heated in air, coloured films are formed as in the case of steel. It has a hardness of 6.5 to 7.5 and a density at 20° C. of 18.72. It melts at 3267°±30° C. and boils at about 5000° Abs. Its heat of evaporation is very high, being about 218000 – 1.8 T gram calories per gram atom. The vapour pressure of tungsten at its melting-point is 0.080 mm. Moissan found that tungsten, like molybdenum and uranium, was very refractory, but might be distilled in the electric furnace. Its specific heat at ordinary temperatures is 0.0358. At high temperatures the atomic heat is almost lineal between 1200° C. and 2400° C., the values at these temperatures being respectively 6.25 and 7.35 calories per gram atom degree. The coefficients of linear expansion increase regularly with temperature; from -100° C. to 0° C. the average coefficient is 4.2×10-6 per degree centigrade, whilst from 0° C. to 500° C. the average value is 4.6×10-6 per degree centigrade. The compressibility of tungsten, which at 20° C. is 0.28×10-6 per megabar, is the smallest of any hitherto studied. Tungsten is non-magnetic. Its magnetic susceptibility between 18° and 1100° C. is + 0.33. Measurements of its electrical resistance at 1000° to 2000° C. have been made. The emission of electrons from heated tungsten has been observed, and also, though only at temperatures above 2500° C., of slow-moving positive ions. By charging a condenser to 30,000 to 40,000 volts and rapidly discharging it through a short tungsten wire of 0.035 mm. diameter in a vacuum, tungsten wire is completely dissipated, no solid residue or smoke remaining. Momentary temperatures of about 20,000° C. are obtained in this way, and it was at first thought that some of the tungsten atoms might be disintegrated in this manner, evolving helium nuclei. Apparently, however, even this violent disturbance is unable to disrupt the atomic system of the element.

The electrical potential of tungsten in solutions of different acids, bases, and salts has been measured against certain standard electrodes at 25° C. The tungsten does not behave as an insoluble electrode, but sends ions into the solutions. Under certain specified conditions - for example, with high-current densities (2 amperes per square decimetre) in aqueous alkalis, but with low-current densities in aqueous solutions of acids and salts - the tungsten anode becomes passive. The passivity appears to be due to adherent films of hydrated oxides. The electrochemical equivalent of tungsten has been found to be 0.3173 mg. per coulomb, which is in close agreement with the theoretical value.

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