Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/497

Rh HYDROXIDES.] CHEMISTRY 485 tion of sodium chloride, for example, may be regarded as containing a certain amount of sodium hydroxide and hydrochloric acid formed by the reaction NaCl + OH 2 = HC1 + NaOH. Sodium chloride. Water. Hydrochloric acid. Sodium hydroxide. Table of Phenomena attending Solution of Salts in Water, Name. Formula. I No. of mols. of water taken. No. of units of heat developed or absorbed per molecule. 1. Crystalline Chlorine, Bromine, and Iodine compounds. Lithium chloride LiCl 200 + 8,440 Sodium chloride NaCl 200 - 1,180 Potassium chloride KC1 ( 200 - 4,440 Sodium bromide NaBr { 100 200 - 4,410 150 Potassium bromide KBr 200 - 5,080 Sodium iodide Nal 200 + 1,220 Potassium iodide KI 200 - 5,110 Magnesium chloride Aluminium chloride 2. Nitrates. Sodium nitrate . ... MgCl, A1 2 C1 6 NaN0 3 800 700 200 + 35,920 + 153,690 - 5 060 Potassium nitrate KNO 3 200 - 8 520 Silver nitrate AcNO. 200 - 5 440 Thallium nitrate T1N0 3 300 - 9 970 Barium nitrate Ba(NO 3 ) 2 400 - 9 400 Strontium nitrate Sr(NO,) 400 - 4 620 Lead nitrate . Pb(NO,), 400 - 7 600 3. Sulphates, f Sodium sulphate { NajS0 4 400 f 50 100 60 - 17,460 - 18,130 Potassium sulphate Magnesium sulphate Zinc sulphate Na. 2 S0 4 + 10H 2 K 2 S0 4 MgS0 4 +7H,0 ZnS0 4 + 7H 2 -{ 200 400 600 400 400 400 - 18,550 - 18,760 - 18,810 - 6,380 - 3,910 4,240 Iron sulphate FeSO 4 + 7H 2 400 - 4,510 Nickel sulphate NiS0 4 + 7H 2 800 4,250 Cobalt sulphate CoS0 4 + 7H 2 800 3,570 The occurrence of reactions of this kind would in many cases involve an absorption, but in others a development of heat. The only two substances mentioned in the above table which develop heat to any extent when dissolved in water, it will be seen, are magnesium and aluminium chlo rides. Both of these, however, are known to form com pounds with water, and both probably enter to a very con siderable extent into reaction with it in the manner above pointed out. It is even probable that the latter cannot exist as such when dissolved in water ; the development of so large an amount of heat is therefore readily understood. But at present we are unable satisfactorily to account for the difference observed between salts such as potassium and sodium iodides, which so closely resemble each other in most respects, one of which, it will be noticed, absorbs 5110 units, whilst the other develops 1220 units of heat when dissolved in water. From this it will be obvious that the study of the con dition of salts in solution is beset with difficulties ; the thermochemical method of investigation appears in most cases to be the only one which is applicable, since the in troduction of new substances at once introduces a new set of conditions, but on account of the complexity of the phenomena attending dissolution, even the results obtained by this method possess only a limited value, and at present only general conclusions can be drawn from them. ^ &quot;Water, as we have already stated, enters into combina tion with oxides of many of the elements, forming two classes of compounds, the acids, and the metallic hydroxides or hydrates. The general properties and rela tions of these two classes of compounds may with advantage now be discussed. The monoxides of the highly positive monad elements caesium, rubidium, potassium, sodium, and lithium form with water easily soluble hydroxides, which cannot be de composed by heat ; their solutions are soapy to the touch, and restore the blue colour to vegetable infusions which have been reddened by an acid. These hydroxides are usually termed alkalies, a solution which has the power of restoring the blue colour to reddened litmus being said to exhibit an alkaline reaction. The term alkali is of Arabic origin, and was at first given to the crude sodium carbonate obtained from the ashes of sea-weed, a solution of which is soapy to the touch and restores the blue colour to reddened litmus, and like the above-mentioned hydrox ides also has powerful cleansing properties. The hydroxides derived from the monoxides of barium, strontium, and calcium, which are less positive elements, also exhibit an alkaline reaction, but they are not nearly so soluble in water ; and, with the exception of barium hy droxide, they are decomposed on ignition, yielding the oxide and water. None of the remaining positive ele ments, except thallium, furnish soluble hydroxides which exhibit an alkaline reaction. Similar differences may be observed between the oxides of negative elements, which furnish acids when combined with water. Thus, the monoxide of the highly negative element chlorine readily dissolves in water, but the acid produced is exceedingly unstable ; similarly, the acids de rived from the oxides of nitrogen are soluble, but of low stability. The oxides of the less negative elements, sulphur and phosphorus, however, furnish very soluble acids, which exhibit considerable stability, being with difficulty resolved into the oxide and water. The lower oxides of the most positive elements enter into combination with water in such proportions as to produce compounds containing an equal number of atoms of hydro gen and oxygen ; for example, sodium monoxide and water furnish sodium hydroxide Xa,O + OH 2 = 2XaOH; Sodium monoxide. Water. Sodium hydroxide. and barium oxide and water yield barium hydroxide BaO 4. QH 2 = Ba0 2 H 2 or Ba(OH),. Barium oxide. Water. Barium hydroxide. Hence these hydroxides may be looked upon as combina tions of the respective elements with the monad compound radicle (OH) or hydroxyl; and the various compounds obtained from the oxides of the remaining elements by the action of water may either be regarded as similarly constituted, or may be viewed as combinations of one or more OH groups with compound radicles formed by the union of the elements with oxygen. Thus, we may regard nitric acid, HXO 3, as a compound of the two monad radicles, (NO 2 ) and (OH) ; sulphuric acid, H. 2 S0 4, as a combination of the dyad radicle (S0 2 ) with two OH groups; and phosphoric acid, H 3 PO 4 , as a combination of the triad radicle (PO) with three monad hydroxyl groups. The number of OH groups which may be associated with a single atom of a given simple radicle or element, or with a compound radicle, entirely depends upon the nature of the radicle, but does not appear ever to exceed four ; the stability of the compound also varies with the radicle, the tendency to form stable compounds with OH being the greater the more positive the radicle. In the case of compounds of feebly positive radicles with several OH groups, there is always a tendency for the elements of one or more molecules of water to separate from the com pound, thus producing a body which is to be regarded as