Sodium

Properties.

Metallic sodium possesses a silvery lustre, but on exposure to moist air the surface is rapidly dulled by a layer of the hydroxide. It may be obtained crystalline by melting it in a sealed tube containing hydrogen, allowing it to cool partially, and then pouring off the liquid portion. The specific gravity is 0.971 at 20° C. At ordinary temperatures the metal has the con sistency of wax and can be readily cut ; on cooling it hardens. On heating it melts at 97° to a liquid resembling mercury, and boils at about 88o° C, yielding a vapour, colourless in thin layers but a peculiar purple, with a greenish fluorescence, when viewed through thick layers. (For the optics of sodium vapour see R. W. Wood, Physical Optics.) Sodium ranks fourth to silver, copper and gold as a conductor of electricity and heat, and is the most electropositive metal with the exception of caesium, rubidium and potassium.

The metal is very reactive chemically. Exposed to moist air it rapidly oxidizes to the hydroxide; and it burns on heating in air with a yellow flame, yielding the monoxide and dioxide; in extremely dry air it is inert. (See DRYNESS, CHEMICAL.) A fragment thrown on the surface of water rapidly disengages hy drogen, which gas, however, does not inflame, as happens with potassium; but inflammation occurs if hot water be used, or if the metal be dropped on moist filter paper. Sodium combines directly, sometimes very energetically, with most non-metallic elements. It dissolves in liquid ammonia giving a blue solution in which its molecular weight is 23 (Kraus)—i.e., the molecules are monatomic. It also combines with dry amp onia at 300-400° to form sodamide, a white waxy mass when pure, which melts at 155°. Heated in a current of carbon dioxide sodamide yields caustic soda and cyanamide, and with nitrous oxide it gives sodium azoimide ; it deflagrates with lead or silver nitrate and explodes with potassium chlorate. Sodamide was introduced by Claisen as a condensing agent in organic chemistry, and has since been applied in many directions. Sodium is largely employed in the manufacture of cyanides and was formerly used in reduction processes leading to the isolation of such elements as magnesium, silicon, boron, aluminium, etc. ; it also finds application in or ganic chemistry. With potassium it forms a liquid alloy resem bling mercury, which has been employed in high temperature thermometers. (See THERMOMETRY.) In its chemical combinations sodium is usually univalent; its salts are generally soluble in water, the least soluble being the pyroantimoniate, the dihydroxytartrate, and the triple nitrite 5Bi(NO2)3, 6NaNO2.

Sodium hydride, NaH, is a crystalline substance obtained di rectly from sodium and hydrogen at about 360°. It burns when heated in dry air, and ignites in moist air; it is decomposed by water, giving caustic soda and hydrogen. Dry carbon dioxide is

decomposed by it, free carbon being produced; moist carbon dioxide, on the other hand, gives sodium formate.

Several oxides have been described, but only two are important. The monoxide, is obtained by heating the metal above 18o° in a limited amount of slightly moist oxygen. It forms a grey mass, which melts at a red heat and violently combines with water to give the hydroxide. The hydroxide or caustic soda, NaOH, is usually manufactured from the carbonate or by elec trolysis of salt solution. (See ALKALI MANUFACTURE.) When anhydrous it is a colourless opaque solid which melts at 318°, and volatilizes with partial decomposition at a white heat. It is very soluble in water, yielding a strongly alkaline solution ; it also dissolves in alcohol. It absorbs moisture and carbon dioxide from the atmosphere. Several hydrates are known; plus is obtained crystalline by cooling concentrated solutions, by allowing the very concentrated solution to crys tallise while hot, and from a solution in 96.8% alcohol.

Sodium peroxide, is formed when the metal is heated in an excess of air or oxygen. In practice the metal is placed on aluminium trays traversing an iron tube heated to 300°, through which a current of air, freed from moisture and carbon dioxide, is passed ; the process is made continuous, and the product con tains about 93% Na202. When pure, sodium peroxide has a faint yellowish tinge, but on exposure it whitens. When dissolved in water it yields some NaOH and H202, but from a cold solution separates as large tabular hexagonal crystals, and this is also obtained by precipitating a mixture of caustic soda and hydrogen peroxide solutions with alcohol. Acids yield a sodium salt and free oxygen or hydrogen peroxide; with carbon, dioxide it gives sodium carbonate and free oxygen ; carbon mo noxide gives the carbonate ; whilst nitrous and nitric oxides give the nitrate. A solution in hydrochloric acid, consisting of the chloride and hydrogen peroxide, is used for bleaching straw under the name of soda-bleach ; with calcium or magnesium chlorides this solution gives a solid product which, when dissolved in water, is used for the same purpose. Sodium peroxide is chiefly em ployed as an oxidizing agent, being used in mineral analysis and in various organic preparations; it readily burns paper, wood, etc., but does not evolve oxygen unless heated to a high tempera ture. Sodyl hydroxide, is said to exist in two forms: one, Na-0.0H, obtained from hydrogen peroxide and sodium ethoxide; the other, 0 :Na.OH, from absolute alcohol and sodium peroxide at o°. They are strong oxidizing agents and yield alka line solutions which readily evolve oxygen on heating.