Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/491

Rh HYDEOGEN.] C H E M IS T R Y 479 various ways. Thus, when two platinum plates, con nected with the poles of a voltaic battery, are plunged into water acidulated with a few drops of sulphuric acid, hydrogen is evolved from the plate in connection with the negative pole, oxygen being disengaged from the positive pole. Water is also resolved into its elements when its vapour is heated by passing through an intensely ignited platinum tube, or by discharging electric sparks in an atmosphere of steam ; but under these circumstances, owing to the recombination of much of the hydrogen and oxygen, only a small quantity of the mixed gases is obtained. Hydrogen may also be obtained from water by the action of the highly positive metals, caesium, rubidium, potassium, sodium, and lithium, at ordinary temperatures ; in each case a solution of the metallic hydroxide is obtained, and hydrogen evolved, thus: 2Xa + 20H 2 = H 2 + 2NaOH. Sodium Water. Hydrogen. Sodium hydroxide. Barium, strontium, and calcium also decompose water in the cold, and hydrogen is evolved when magnesium is heated with water at temperatures below the boiling point of the latter. Many other metals, such as iron, manganese, zinc, cadmium, cobalt, nickel, tin, and antimony evolve hydrogen from water when its vapour is passed over the metal heated to redness, the oxide of the metal being formed, thus : 3Fe Iron. 40H 2 = 4H 2 Water. Hydrogen. f6IT ^ niC Aluminium also decomposes water at a red heat, but owing to the formation of an impermeable coating of aluminium oxide on the surface of the metal the action soon stops. Metals like copper, mercury, silver, gold, and platinum are without action even at a bright red heat. Many metals, however, which do not decompose water unless heated with it, if placed in contact with a more negative element cause the evolution of hydrogen at ordinary atmospheric temperatures ; for example, if a plate of zinc, coated with spongy copper by immersion in a solution of copper sulphate, be placed in water, hydrogen is gradually evolved, the reaction which occurs being as follows : Zn + 20H 2 = H 2 + Zn(OH) 2 Zinc. Water. [Hydrogen. Zinc hydroxide. Hydrogen is usually prepared by the action of zinc or iron on a solution of hydrochloric or sulphuric acid. The change which occurs is represented in the following equa tions : Zn f- 2HC1 = H 2 + ZnC Lj Zinc. Hydrochloric acid. Hydrogen. Zinc chloride Zn + H 2 S0 4 = H 2 + ZnS0 4. Zinc. Sulphuric acid. Hydrogen. Zinc sulphate. All metals which readily decompose water w T hen heated readily furnish hydrogen on treatment with hydrochloric and sulphuric acid, and many other metals enter more or less readily (although none so readily) into reaction with these acids ; also many other acids may be used in place of hydrochloric or sulphuric acid, but none act more readily. In all cases the action consists in the displacement of the hydrogen of the acid by the metal employed, and if the acid is not one which can enter into reaction with the dis placed hydrogen, the latter is evolved as gas. If pure hydrogen is required, it is necessary to employ pure zinc or iron ; the impurities in the ordinary metal communicate an extremely disagreeable odour to the gas. On the large scale nearly pure hydrogen may be pre pared by passing steam over charcoal or coke heated to dull redness. If the temperature be kept sufficiently low, hydrogen and carbon dioxide are the sole products : C + 2H 2 O-=2H 3 +C0 2 and the latter may be removed by causing the gas to traverse a vessel filled with slaked lime ; but if the tem perature be allowed to rise too high carbon monoxide is also produced, and cannot be removed from the mixture. Pure hydrogen is a colourless, transparent, odourless and tasteless gas. It has never been liquefied, and is very slightly soluble in water, 100 volumes of water dissolving 1 93 volumes, of the gas at all temperatures between O 9 and 24 C. It is the lightest of all known bodies, its specific gravity being 0693, that of air being unity; one litre of hydrogen at C, and under the pressure of 760 mm. of mercury, weighs 0896 gramme, and it is impor tant to remember this number, since the weight of a litre of any other gas may be at once found by multiplying 0896 by half the molecular weight of the gas, the specific gravity of a gas referred to hydrogen being always half its molecular weight (p. 471). Pure hydrogen is not poisonous, though it cannot sup port life; and if mixed with a certain proportion of oxygen, it may be breathed for some time without inconvenience. It is extremely inflammable, and burns in the air with a colourless non-luminous flame forming water; a burning taper is extinguished when plunged into hydrogen, and all bodies which burn in the air are incapable of burning in hydrogen. Hydrogen does not spontaneously enter into reaction with any of the elements, although it has a powerful affinity for several of them. Thus, when hydrogen and oxygen are mixed nothing occurs, but if a burning taper or a heated wire is applied, a violent explosion ensues, water being produced. Similarly, chlorine and hydrogen are without action upon each other in the dark, but if the mixture is exposed to a bright light or is heated by the passage of an electric spark, the gases at once com bine with explosive violence, forming hydrochloric acid. It has already been pointed out that we must suppose that the molecules which constitute free hydrogen, chlorine, and oxygen gases are diatomic, and that hydrogen and chlo rine, for example, do not directly combine to form hydro chloric acid, but that an action occurs such as is represented by the equation and that, therefore, the combination of the atoms of chlo rine and hydrogen is preceded by the separation of the two atoms of chlorine in the chlorine molecules, and of the two atoms of hydrogen in the hydrogen molecules, from each other, which necessarily involves an expenditure of energy. The application of heat, we may assume, serves to effect this decomposition of some of the molecules of hydrogen, oxygen, and chlorine into their constituent atoms, and since much heat is evolved by the subsequent combination of the atoms of hydrogen and chlorine, or of hydrogen and oxygen, it is only necessary to apply heat to start the re action, because the remaining molecules are decomposed by the heat developed in the formation of the first portions of water or hydrochloric acid. Compounds of hydrogen with bromine, iodine, sulphur, and nitro gen may also be obtained directly from these elements and hydrogen, although only with difficulty and in small quantity. OXYGEN. Symbol, ; Atomic wt., 15 96 ; Molecular wt., 31 92 ; Valency,&quot;. Oxygen was first isolated by Priestley in 1774 ; its name is derived from 6v?, sour, and yewdw, to generate, in allu- ion to the circumstance that many of the bodies formed by combining it with other elements dissolve in water, pro ducing sour or acid solutions. It was long believed, in fact, that oxygen was a constituent of all acids.
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