HYDROXIDES.] neutralization of various hydroxides insoluble in water by a molecule of sulphuric acid Magnesium hydroxide 31,220 Manganese ,, 26,480 Nickel ,, 26,110 Cobalt ,, 24,670 Iron ,, 24,920 Cadmium 23,820 Zinc ,, 23,410 Copper ,, 18,440 Thomsen, however, considers that the differences be tween the amounts of heat developed when these hydrox ides are dissolved in acids, and when soluble hydroxides are neutralized, are due to the heat absorbed in rendering them soluble ; or in other words, that solutions of these hydroxides would have the same heat of neutralization as solutions of soluble hydroxides. One of the most interesting results of the thermochemi- cal investigation of the behaviour of acids with metallic hydroxides is the proof that when sodium hydroxide, for example, is added to a mixture of two acids in insufficient quantity to neutralize both, the sodium salts of the two acids are seldom formed in the proportions in which the two acids are mixed. Thus, when a solution of 2 mole cules of sodium hydroxide is mixed with a solution con taining two molecules of monobasic nitric acid and one molecule of dibasic sulphuric acid, two-thirds of the sodium hydroxide enter into reaction with the nitric acid and one-third with the sulphuric acid. The avidity, as it is termed by Thomsen, of nitric acid to enter into reaction with sodium hydroxide is thus twice as great as that of sulphuric acid. The avidity of hydrochloric acid for sodium hydroxide is equal to that of nitric acid, but the avidities of hydrobromic, hydriodic, and hydrofluoric acids are less than that of hydrochloric acid in the proportion HC1 : HBr : HI : HF = 100 : 89 : 79 : 5. In concluding this brief account of the thermochemical behaviour of some of the acids and hydroxides, we may observe that the interpretation of most of the remarkable results to which we have alluded has not been given. The study of this branch of chemistry is yet in its infancy, and is beset with difficulties, but its importance can scarcely be exaggerated. At present we scarcely know more than that in the formation of a given substance a certain amount of heat is finally developed or absorbed, but in most cases we are ignorant of the value of one or more of the several distinct operations which we believe are involved in the reaction by which it is produced, so that the results given by different substances cannot as a rule be satisfactorily compared. Many substances belonging to all classes of chemical compounds unite with water, forming combinations from which it may be again expelled by the application of a greater or less degree of heat, and from the circumstance that many of these combinations are crystalline, the water is said to be present in the form of u ater of crystallization. Thus, potassium hydroxide crystallizes with 2 molecules of water, as KHO + 2H 2 O ; copper sulphate with 5 mole cules, as CuSO 4 + 5H O ; and sodium sulphate with 7 and with 10 molecules. The water may usually be ex pelled from these compounds by the application of a gentle heat, and many salts part with their water of crystalliza tion on exposure to the air ; but others are not deprived of it, or more than a part, unless very strongly heated. Copper sulphate, for example, readily loses 4 of the 5 molecules of water with which it combines, but the remain ing molecule is only expelled by heating to about 200 C. 5 of the 7 molecules of water in crystallized magnesium sulphate are readily removed, and the sixth is expellee at about 150 C., but the seventh is retained at 200 C. 489 The compounds containing water of crystallization are usually denied the title of atomic compounds ordinarily applied to combinations of two or more elements in which .t is supposed the constituent atoms are all associated in a single molecule under the influence of the force which has received the name of chemical affinity, and in contra distinction are termed molecular compounds, being regarded as combinations of two or more separate molecules. This, although perhaps true of many of the compounds contain ing water of crystallization, is certainly not true of all, and notably of the sulphates which are only deprived of their water by heating to high temperatures. Hydrogen Dioxide, H 2 2 = 33 92. This compound cannot be obtained directly from its elements, but there appears to be little doubt that it may formed by the combination of oxygen with water, since it is produced in many cases of slow oxidation in presence of water, and particularly in processes where ozone is formed. It is produced in small quantity when water is decomposed by an electric current, especially when small lectrodes are employed. Its formation in this manner may be regarded as an oxidation of water, but it has been suggested that it is the direct product of electrolysis, and that the oxygen evolved is a secondary product derived
- rom the decomposition of the hydrogen dioxide ; thus
2H 2 O =H 2 + H 2 0. 2 ; 2H 2 O 2 =O 2 +2H 2 0. It is usually prepared from barium dioxide by double de- omposition with hydrochloric or carbonic acid : Ba0 2 + 2HC1 = H 2 0. 2 + BaC) 2 . Barium dioxide. Hydrogen chloride. Hydrogen dioxide. Barium chloride. Hydrogen dioxide or peroxide is an exceedingly un- table substance, and readily decomposes even in aqueous solution into water and oxygen, especially on heating. The solution is more stable if slightly acid. A dilute solution may be concentrated by evaporation in vacuo over sul phuric acid, and hydrogen dioxide was obtained in this manner by its discoverer Thenard as a colourless trans parent liquid, of specific gravity 1 452, which did not freeze at - 30 C. ; it is doubtful, however, whether it has ever been prepared quite free from water. It did not redden litmus, but had a harsh bitter taste ; when placed upon the hand it instantly turned the cuticle white. Hydrogen dioxide exhibits the closest resemblance to ozone. Thus, it is decomposed by mere contact with finely- divided metals, such as silver, gold, and platinum, with evolution of oxygen. Like ozone it is a powerful reducing agent, entering into reaction with silver oxide, for example, to form water, oxygen, and metallic silver : According to Fairley, about 37,000 units of heat are developed in the production of 32 grammes of oxygen by this reaction, which thus affords further proof of the strength of the affinity of oxygen for oxygen, as probably the heat developed is chiefly, if not entirely, due to the combination of the oxygen atoms. Many other oxides are reduced by it, either to the metallic state or to lower oxides, oxygen being evolved and water produced. But hydrogen dioxide is also a powerful oxidizing agent. Thus, it decolorizes a solution of indigo ; it con verts sulphurous acid, HoSO 3 , into sulphuric acid, H 2 SO 4 ; lead sulphide, PbS, into lead sulphate, PbS0 4 ; and many oxides into higher oxides. It decomposes a solution of hydriodic acid with separation of iodine : With the aid of this reaction an important series of ex periments to ascertain the connection between the con-
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