Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/498

Rh 486 CHEMISTRY [ACIDS, BASES, a combination of one or more OH groups with an oxygen ated radicle. For instance, ferric hydroxide, Fe 2 (OH) (5, very readily loses water, forming compounds such as Fe 2 O(OH) 4 and Fe 2 O 2 (OH) 2 . Nearly all the bodies produced by the action of water on the oxides of the non-metals are to be regarded as compounds of oxygenated radicles with hydroxyl, and are more or less acid in character. As a rule, the more negative the radicle with which OH is associated the more powerful will be the acid, aud since the addition of oxygen to the radicle renders it more negative, the acid furnished by the higher oxide of an element is usually more stable, and also a more powerful acid, than that furnished by the lower oxide ; thus, sulphurous acid, H 2 S0 3 , is extremely unstable as compared with sulphuric acid, H 2 S0 4 , and a much less powerful acid, but the former may be regarded as a com bination of the dyad radicle (SO), and the latter as a com bination of the dyad radicle (SO.,), with twice the monad radicle (OH). The behaviour of the positive elements is exactly complementary of this, since the greater the amount of the negative element oxygen associated with them the less is the tendency to furnish corresponding hydroxides when combined with water, and the less the stability of the resulting hydroxides ; and, as a rule, also the basic pro perties both of the oxides and of the hydroxides correspond ing to them become much less pronounced as the relative proportion of oxygen increases. In all cases in which oxides unite with water to produce stable combinations, much heat is developed ; the stability of the compounds formed by the combination of water with oxides, in fact, appears to be directly in proportion to the amount of heat developed in their formation, and it has been shown that the formation of many of the most unstable hydroxides from their elements would be accom panied by an absorption of heat ; hence their instability is readily understood. When the hydroxides are added in sufficient quantity to solutions of the acids, mostly neutral solutions are pro duced, that is to say, solutions which do not affect either blue or red litmus. The acid is then said to be neutral ized by the hydroxide, or vice versa. The basic oxides which furnish hydroxides, and indeed the basic oxides generally, behave in a similar manner with acids. In these cases a salt is produced ; thus, by the addition of sodium hydroxide to sulphuric acid the salt sodium sulphate is ob tained 2NaOH Sodium hydroxide. H 2 S0 4 = Na 2 S0 4 + 2H 2 0. Sulphuric acid. Sodium sulphate. Water. Obviously, we may regard the salts produced in this man ner as formed by the displacement of the hydrogen of the acid by more or less positive elements or simple radicles, and they may actually in many cases be prepared by the action of the metals on the acids ; for example H 2 S0 4 = 2 ZnS0 4 Zinc. Sulphuric acid. Hydrogen. Zinc sulphate. H They are also obtained, as we have seen, by the union of a more or less basic with a more or less acid oxide. But a large number of salts are known derived from the acids by the displacement of hydrogen by compound radicles ; thus, many of the salts of the element vanadium are formed by the introduction of the group (V 2 2 ) IV, which functions as a tetrad element, in place of the hydrogen of acids, vanadyl sulphate being (V 2 2 ) 1V (S0 4 ) 2. The ammonium salts are formed in a similar manner by the displacement of hydrogen in acids by the monad compound radicle (N V H 4 ). But the hydrogen of many hydroxides may also be displaced by positive elements or radicles; for example, zinc hydrox ide, although insoluble in water, dissolves in solutions of the alkalies, and on heating metallic zinc with a concen trated solution of potassium hydroxide, it dissolves with evolution of hydrogen Zn + 2HOK = Zn&quot;(OK) 2 + H 2. Zinc hydroxide and similar compounds, therefore, display both basic and acid functions ; and it is difficult to deny the compounds formed from them by introducing positive elements in place of hydrogen the title of salts. Another class of oxy-salts are formed by the union of two different oxides of the same element ; the compounds of fluorine, chlorine, bromine, and iodine with positive elements, and of acid with basic sulphides, are also termed salts, the compounds of sulphur being distinguished as sulpho-salts or thio-salts, whilst those of the four other elements are distinguished as haloid salts. From this it will be obvious that the term salt is of very wide application, and that it is almost impossible to define it in a few words. It is also extremely dim&quot; cult strictly to define an acid, for, although the very greatest difference is observable between the compounds of the oxides of highly negative and of highly positive elements with water, the differences become less and less marked as we pass from one end of the series to the other. The only definition which really separates bodies which are usually regarded as true acids from hydroxides pos sessing acid properties is afforded by the fact that, whilst the hydrogen in all hydroxides which exhibit basic pro perties may be displaced by negative and also in some instances by positive radicles, the hydrogen in acids can only be displaced by positive ralicles. For example, the hydrogen in zinc hydroxide may be displaced by the posi tive radicle potassium and by the negative radicle NO 2, but it is only possible to displace the hydrogen in sulphuric acid by positive radicles. This definition is not applicable, however, to organic acids. The extent to which hydrogen may be displaced in an acid by positive radicles is termed its basicity, an acid which contains a single atom of displaceable hydrogen being termed monobasic, and acids containing two or three atoms of displaceable hydrogen, dibasic or tribasic. On the other hand, the hydroxides and basic r.xides are frequently spoken of as monacid, diacid, or triacid, &amp;lt;fcc., according as they are capable of neutralizing a single molecule, two, or three molecules of a monobasic acid ; thus, sodium hydroxide is monacid since a single molecule neutralizes a single molecule of monobasic nitric acid, forming the salt sodium nitrate NaOH + HN0 3 = NaN0 3 + OH 2 , Sodium hydroxide. Nitric acid. Sodium nitrate. Water. and barium hydroxide is a diacid base since it neutralizes two molecules of nitric acid Ba0 2 H 2 + 2HN0 3 = Ba(NO 3 ) 2 -f 20H 2. Barium hydroxide. Nitric acid. Barium nitrate. Water. But we may also regard the salts formed by the action of acids on hydroxides as derived from the latter by the displace ment of hydrogen by negative compound radicles, sodium nitrate, for example, as sodium hydroxide in which the atom of hydrogen has been displaced by the monad com pound radicle N0 2 ; hence we may define the acidity of hydroxides to be the extent to which hydrogen may be displaced in them by negative radicles. In polybasic acids, that is to say, in acids containing two or more atoms of displaceable hydrogen, it is possible to displace the hydrogen step by stop ; thus, by the addi tion of a single molecule of sodium hydroxide to a single molecule of sulphuric acid, only one-half the hydrogen in the acid is displaced NaOH + H 2 S0 4 - NaIIS0 4 + OH 2 Sodium hydroxide. Sulphuric acid. Sodium hydrogen sulphate. Water. but by the addition of a second molecule of the hydroxide the second atom of hydrogen is also displaced