HYDROGEN (Greek, °producing water" in reference to the fact that in burning in air or oxygen, hydrogen forms water-vapor), a gaseous element, discovered by Cavendish in 1766. It was at first called "inflammable air,x' the present name being due to Lavoisier. Hy drogen is the lightest known substance, and it also has the smallest known atomic weight. The atomic weight of hydrogen is often taken as unity in stating the relative atomic weights of the different elements (see Aromic THEORY ) , but it is now more usual to assume the atomic weight of oxygen to be precisely 16, which amounts to making the atomic weight of hydro gen 1.0076. According to Regnault's experi ments, hydrogen has a density equal to 0.06926 of that of an equal volume of air, at the same temperature and pressure. In absolute measure, the mass of a cubic centimetre of hydrogen, at the freezing point of water and under a pres sure of 76 centimetres of mercury at Paias, is 0.00008957 gram, or 0.08957 milligram. Hydro gen has the chemical symbol H, and is one of the most abundant elements known. It occurs in the free state in volcanic gases, and in the sun and in many of the fixed stars. Free hy drogen is also present in the earth's atmosphere in exceedingly small amount. Water (or hy drogen monoxide) is its commonest and most abundant compound, and it is an essential con stituent of nearly all organic tissues. Hydro gen may be prepared very easily by many methods. One of the most convenient of these consists in acting upon metallic zinc with dilute sulphuric acid, the reaction in this case being: Zn + H:SO4=ZnSO4+ 2H. Hydrogen is chem ically inert toward most of the elements, at ordinary temperatures, but it combines with chlorine when exposed to light,— quietly in diffused daylight, and explosively under the direct action of sunlight. At elevated temper atures it combines with other elements also, and it burns in air (or oxygen) with the develop ment of an intense heat, but with very little light; water being produced as the result of the combustion. Hydrogen has been both liquefied and solidified.. Its critical temperature is esti mated, by Dewar, to be about 402° F. below zero, and the same authority gives 15 atmo spheres as the critical pressure. See CRITICAL POINT, and GASES, LIQUEFACTION OF.
Hydrogen combines with oxygen in two pro portions. The monoxide, or common water, 11.0, is formed, as already noted, when hydro gen is burned in air or in oxygen. It is also formed in many of the double decompositions that occur in chemistry, as when metallic oxides or hydrates are dissolved in acids. Sodium hydrate, for example, combines with sulphuric acid according to the equation 2NaOH + 11,S0.= Na,SO4 2H2O, sodium sulphate and
water being formed.
Hydrogen peroxide (or dioxide), H2O., may be prepared by acting upon barium dioxide, BaO,, with dilute sulphuric acid; the reaction being: BaO, HiS0.= BaSO4 + H20,. The barium sulphate that is formed at the same time is a heavy, insoluble substance, which is easily removed from the solution by filtration. or by settling and subsequent decantation. The aque ous solution of the peroxide may then be con centrated by evaporation over strong sulphuric acid, under the receiver of an air pump. When the water has all evaporated, the pure peroxide remaining behind has a specific gravity of 1.452. and is a colorless, oily liquid, devoid of odor, but having a disagreeable metallic taste. The peroxide does not freeze, even when cooled to 0° F. At 70° F. it slowly gives off half its. oxygen, passing into water. At 212° F. this change takes place very rapidly. Owing to the facility with which hydrogen peroxide gives off oxygen, it is used quite largely as a bleaching agent, and also, in surgery, as a disinfectant in the treatment of wounds.
When hydrogen is passed through boiling sulphur, combination takes place, with the formation of hydrogen sulphide, or sulphuretted hydrogen, H.S. This compound is more con veniently prepared, however, by treating sul phide of iron, FeS, with dilute sulphuric acid, ferrous sulphate (FeSO4) being formed at the same time. The reaction is: FeS H.SO4= FeSO. H.S. Sulphuretted hydrogen is a gas, devoid of color, but possessing an overpowering odor, suggestive of rotten eggs. It burns with a bluish flame and is poisonous when inhaled in any considerable quantity, even though largely diluted with air. Under ordinary at mospheric pressure, sulphuretted hydrogen gas condenses, at F. below zero, to a colorless liquid, which freezes to an ice-like solid upon being further cooled to 121° F. below zero. Liquefaction may also be induced at the ordi nary temperature of the air by the application of a pressure of from 17 to 20 atmospheres. Sulphuretted hydrogen is an invaluable reagent in the chemical laboratory, where it is greatly used for separating the metals into groups, in inorganic analysis. See CHEMICAL ANALYSIS.
With carbon, hydrogen forms a large num ber of compounds which are collectively known as hydrocarbons (q.v.). With carbon and oxy gen, and with carbon, oxygen and nitrogen, hydrogen forms compounds past enumeration. For further information concerning these, con sult any treatise on organic chemistry, and also, in this encyclopaedia, the articles COMPRESSED