Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/495

Rh WATER.] CHEMISTRY 483 oxidized, and Brodie s experiments show that four classes of oxidations may be effected by ozone : In the first class of reactions one-third of the molecule is active in effecting oxidation, the remaining two-thirds being obtained as oxygen, so that the volume of oxygen ob tained is the same as the volume of ozone operated upon, If the number of atoms of oxygen active in effecting oxida tion are placed within brackets, this class of oxidations is expressed by the equation The action of ozone on a solution of sodium dioxide, quoted above, and its action on a solution of potassium iodide 2KI + OH., + O 3 = O 2 + I. 2 + 2KHO, Potassium ^ Qzone. Qiygen. Iodin, P*lnm on ferrous chloride and sulphate, on an acid solution of potassium ferrocyanide, and on sodium arsenite, are special cases of this class of oxidations. In the second class of reactions, four atoms of oxygen from two molecules of ozone are active, the remaining two atoms being obtained as oxygen ; or 20 3 = O 2 + 4(0) ... II. The oxidation of strongly alkaline solutions of sodium hyposulphite, the oxidation of hydriodic acid, and the oxidation of barium, pentasulphide are included in this class. In a third class of reactions, ten atoms of oxygen from four molecules of ozone are active in promoting oxidation, a single molecule of oxygen being obtained ; or 40 3 = O. 2 +10(0) .... III. The oxidation of sodium sulphydrate and of hydriodic acid at C. are to be referred to this class. Lastly, in a fourth class of reactions, the entire molecule is active in effecting oxidation ; or 3 = 3(0) ..... IV. Examples of this class are afforded by the oxidation of tin dichlori.de, oil of turpentine, and neutral and slightly alkaline solutions of sodium hyposulphite. That ozone can thus enter into reaction in so many different ways affords the most conclusive evidence of its compound nature, and by these reactions it may be dis tinguished from all known gases, and estimated. The reaction with potassium iodide is usually employed for the detection of ozone, the liberation of iodine being rendered evident by the addition of starch paste, which with iodine furnishes a beautiful blue colour. The libera tion of iodine from potassium iodide is not alone sufficient proof, however, of the presence of ozone. By comparing at different times the depth of colour produced on exposing slips of paper moistened with starch paste and potassium iodide solution to the air, for the same length of time, an estimate of the relative amounts of ozone present may be obtained. Compounds of Hydrogen with Oxygen. Water. Symbol, OH S ; Molecular vt., 17 96. Hydrogen and oxygen when mixed do not enter into reaction at the ordinary temperature ; but the contact of flame or of any other red-hot body, the passage of the electric spark, and the contact of platinum cause reaction t;&amp;gt; take place with explosion. The flame and the electric spark act by their intense heat, and the nature of the influence which, perhaps, heat exercises in inducing the combination of hydrogen and oxygen has already been pointed out ; but the action of platinum is more obscure. Spongy platinum and the fine powder of that metal called platinum black, although cold, cause the mixed gates to explode as readily as flame does. Even polished slips of platinum, if perfectly clean, will cause the reaction to take place, although more slowly. It appears that at first the absorption of the gases on the surface of the cold metal, and the consequent approximation of their particles, is sufficient to induce the combination of a part of the gases ; this being accompanied by the development of heat, the metal becomes warmed and acts more vigorously, and the combination of further quantities of the gases takes place, by which more heat is developed, so that by degrees the metal becomes red-Lot, and if any of the mixed gases be still uncombined it causes them to explode. In the case of the powder or the sponge, especially the former, all this takes place so rapidly from the enormous surface of the metal, that it becomes red-hot as soon as it is introduced, and fires the mixture as rapidly as a flame. Now, it is well known that metallic platinum, and especially spongy platinum, or platinum black, readily condenses gases upon its surface ; but the condensation is attended by the de velopment of heat, and it seems not improbable, therefore, that its action in causing the combination of hydrogen with oxygen is of the same character as the action of flame, that the platinum, in fact, does not itself induce the combination, but that the heat resulting from the con densation of the gases by the platinum is the direct cause of the reaction. Graham s researches have shown also that heated platinum absorbs hydrogen, and the absorp tion is doubtless accompanied by the development of heat ; this is an additional reason for the action of platinum becoming more vigorous as it becomes warmer. A series of experiments by Von Meyer (Journal fur praktische Chemie, 1876, p. 121) may here be referred to, as illustrating in a remarkable manner the influence of finely-divided platinum in causing the combination of gases. It was first shown by Bunsen that when a mixture of hydrogen and carbon monoxide is exploded with insuffi cient oxygen to convert these two gases respectively into water and carbon dioxide, whatever the proportions em ployed, the quantities oxidized are always iu a simple molecular ratio. From Von Meyer s experiments it ap pears that, when a similar mixture is placed in contact with finely-divided platinum, although the oxidation is effected very slowly, the same law obtains. Whereas, however, when the mixture of the three gases is exploded, always relatively more hydrogen than carbon monoxide is oxi dized, the proportion in which the two gases are burnt being on the average about as 3 to 1, the reverse is the case when their oxidation is slowly effected with the aid of platinum ; in the latter case the number of molecules of carbon mon oxide oxidized to the dioxide is from 7 to 8 times as great as the number of hydrogen atoms oxidized to water. When oxygen and hydrogen, from whatever cause, enter into reaction, it is always in the proportions to form water, that is, invariably in the proportion of 2 volumes of hydrogen gas to 1 volume of oxygen gas. Any excess of either is left uncombined. If the water which is formed be measured as steam or gas, and its volume compared with the volume of the mixed gases at the same tempera ture before combination, it is found that condensation has taken place, 2 volumes of water gas being obtained from 2 volumes of hydrogen and 1 volume of oxygen gas ; hence 2H 2 Hydrogen. 0. 2 = 2H,O. Oxygen. Water. In the liquid state, however, the volume of the water formed is so small as compared with the volume occupied by the mixed gases that it may be neglected, so that if the contraction in volume which occurs on explosion is measured at the ordinary temperature, two-thirds of tha