rilK.MISTRY 



151 



between what are culled anhydrous bases and 

 h i/i I rated bases or hydroxides. The oxide of lead, 

 PbO, is HII anhydrous base ( or basic oxide ), whilst 

 the compound oliiained liythe action of water upon 

 calcium oxide, CaO (a DAaio oxide, and the only 

 compound of calcium ami oxygen known), is culled 

 u hydrated buse (or hydroxide). The formation of 

 the latter is represented by the equation 



CaO + H !l O = Ca(HO) J . 



The oxides produced from non-metallic elements 

 are very frequently acid oxides i.e. oxides which 

 unite with water to form the class of bodies called 

 Acids (q.v.). The oxides themselves are often 

 called acid anhydrides, whilst the compounds pro- 

 duced by the action of water upon them are called 

 acids, or hydrogen salts. When phosphorus burns 

 in air, phosphoric anhydride, P-jOs, is obtained. 

 This is a white solid substance which unites with 

 water with the evolution of much heat to form a 

 solution of metaphosphoric acid, or hydrogen meta- 

 phosphate : 



P 8 B + H.,O = 2HPO 8 . 



There are a few acids known which do not con- 

 tain oxygen, and are not obtainable by the com- 

 bination of an oxide with water. Examples are 

 hydrochloric acid, HC1, hydrobromic acid, HBr, 

 and hydrocyanic acid, HCN. These are also called 

 hydrogen chloride, bromide, and cyanide respec- 

 tively. 



The two classes of substances, bases and acids, 

 are nearly related to the very large class of salts. 

 A salt is a compound which can be obtained, 

 amongst other ways, by the action of an acid upon 

 a base, water being almost invariably eliminated 

 at the same time ; and just as we saw that the pro- 

 perties of two elements are totally different from 

 those of the compound formed by their combination, 

 so we find that in the formation of a salt the 

 properties of both acid and base to a great extent 

 or altogether become neutralised and disappear. 



If to a solution in water of potassium hydroxide, 

 KHO (which is a powerful base), we add a suffi- 

 cient quantity of nitric acid, HN0 3 , that is until 

 the liquid on thoroughly mixing does not possess 

 either the acid or the alkaline reaction, we obtain 

 a solution in water of potassium nitrate (saltpetre), 

 and nothing else the water eliminated in the 

 action simply mixing with that which is already 

 present : 



KHO + HN0 3 = KN0 3 + H 2 O. 



Acids have already been mentioned as hydrogen 

 salts. The above equation shows how hydrogen 

 nitrate is exactly comparable with potassium 

 nitrate an atom of potassium taking the place of 

 an atom of hydrogen and a characteristic of all 

 hydrogen salts, or acids, is that they contain hydro- 

 gen, which is capable of removal and of having its 

 place thus taken by an equivalent quantity of 

 another metal. In the example above mentioned 

 every 1 part by weight of hydrogen has its place 

 taken by 39 parts by weight of potassium. These 

 quantities of hydrogen and of potassium are equiv- 

 alent, Iwth being capable of uniting with the 

 group NO 3 . This group is an example of what 

 is called a compound radical i.e. a group of 

 elements which is capable of going as a whole 

 through a series of changes. Acids which contain 

 in their molecule one atom of hydrogen replaceable 

 by another metal are called monobasic acids. 

 Nitric acid is thus a monobasic acid, whilst sul- 

 phuric acid, HjSO^ is dibasic, orthophosphoric acid, 

 HsPO 4 , is tribasic, and so on. 



Bases, likewise, are sometimes spoken of as 

 monacid, diacid, triacid, and so on, according as 

 one molecule of the base requires one, two, three, 



.Vc. molecules of a monobasic acid (an nitric acid) 

 t i'. mi what is called a normal Bait, that in, a 

 salt in which all the replaceable hydrogen lian 

 IH-.-II replaced I iy another metal. Thus pita.-.-iuiu 

 hydroxide, KHO, i> a monaeid l.a-e ; calcium 

 hydroxide, or slaked lime, t.t'llO).., is diacid; 

 bismuth hydroxide, I ;i( HO),, is triacid, and BO on. 

 Equations may make this clearer (see the equa- 

 tion above for a monacid base ) : 



Ca(HO), + 2HNO, = Ca(NO,), + 2H-O : 

 Bi (HO) S + 3HNO 3 = Bi (NO,) 3 + 3H,O. 



Salts are formed in many cases by the replace- 

 ment of only a part of the replaceable hydrogen of 

 a hydrogen salt by another metal. Such are called 

 in-ill salts, and K 1 1 sn, is an example. This salt, 

 KHSO 4 , may be looked upon as intermediate be- 

 tween the acid, H 2 SO 4 , and the normal salt, 

 K.,SO 4 . 



Many salts are known which may be looked upon 

 as bases which have their basic character only par- 

 tially neutralised by an acid. Such salts are called 

 basic salts, and as examples may be mentioned 

 BiONO 3 and Pb(OH)NO 3 . The former is inter- 

 mediate between the normal nitrate, Bi(NO,),, 

 and the oxide, BLO 3 , the latter between the nor- 

 mal nitrate, PbtNO,),,, and the hydrate, Pb(OH),. 

 Such basic salts are often produced by the action 

 of water upon the normal salts, as, for instance, 

 in the case of the basic bismuth nitrate : 



Bi(NO 3 ) 3 + H 2 O = 2HNO 3 + BiONO,. 



Salts are looked upon as being composed of metal 

 and salt radical, the latter name being given to all 

 of the salt that is not metal. Thus SO 4 is the salt 

 radical of the sulphates, NO 3 the salt radical of the 

 nitrates, &c. This way of looking at salts arises 

 from the phenomena observed when salts are 

 decomposed by Electrolysis (q.v.), metal and salt 

 radical being the primary products of decom- 

 position. 



Chemical Nomenclature. Chemists endeavour to 

 make the nomenclature of compound substances as 

 systematic as possible, and a certain amount of 

 system has even been introduced into the nomen- 

 clature of the elements themselves. The oxides of 

 the metals are named after the metal which they 

 contain, as magnesium oxide, MgO ; aluminium 

 oxide, ALjO, ; and the series of salts derivable from 

 these oxides are similarly named after the metal. 

 Thus MgCl 2 is magnesium chloride, and AJ 2 (SO 4 ) S 

 is aluminium sulphate. When a metal forms more 

 than one basic oxide, adjectival terminations are 

 employed to distinguish these ; thus the two basic 

 oxides of iron are named ferrous and ferric oxides 

 ( FeO and FeiO 3 ) respectively, and correspondingly 

 there are ferrous and ferric salts. FeSO, is ferrous 

 sulphate ; Fe.,Cl 6 is ferric chloride. Acia salts and 

 in general salts which contain more than one metal 

 are named after the metals which they contain, the 

 compound radical NH 4 ( ammonium ; see AMMONIA ) 

 being regarded as a metal for purposes of nomen- 

 clature. Thus, KHSO> is potassium hydrogen 

 sulphate, whilst HNaNH 4 PO 4 is hydrogen sodium 

 ammonium orthophosphate. 



The nomenclature of non-basic metallic oxides 

 has been rendered systematic by the use of names 

 descriptive of the number of atoms of metal and of 

 oxygen contained in the oxide, as, for instance, 

 trimanganic tetroxide for Mn s O 4 . A considerable 

 number of non- basic oxides, as BaO,, PbO,, MnO,, 

 &c., are somewhat less systematically called per- 

 oxides. 



The acid anhydrides, which, as has already been 

 stated, are oxygen compounds or oxides of the non- 

 metallic elements, are named after the elements of 

 which they are oxides. As there are frequently 

 two or more such acid anhydrides derived from one 



