118 PRINCIPLES OF CHEMISTRY. 



hydrofluoric acid, HF, or H 2 F 2 , hydrogen sulphide, H 2 S, hydrocyanic acid, 

 H(CN>. 



However much the acids may differ in certain properties, such as consist- 

 ency, that is, whether solid, liquid, or gas, solubility in water, degree of acid 

 taste and action on litmus paper, corrosiveness to organic matter, such as skin, 

 wood, cloth, etc., they are all alike in one respect, namely, in containing hy- 

 drogen which is separable from the rest of the molecule, and replaceable by 

 metals, either by direct action of a metal on the acid, as when zinc acts on a 

 solution of sulphuric or hydrochloric acid, or in a round-about way. There 

 seems to be a strong tendency to separation between the hydrogen and the rest 

 of the molecule of an acid which remains intact as a unit. 



According to the number of hydrogen atoms replaceable by metals, we dis- 

 tinguish monobasic, dibasic, and tribasic acids. Hydrochloric acid, HC1, is a 

 monobasic ; sulphuric acid, H 2 SO 4 , is a dibasic ; phosphoric acid, H 3 PO 4 , is a 

 tribasic acid. 



Many of the acids sold in trade, as well as the reagents used in the labora- 

 tory, are solutions of acids in water. It is customary to call these solutions 

 by the names given to the acids themselves. 



Bases or basic substances show properties which are chemically 

 opposite to those of acids. As a general rule bases are compounds 

 of electro-positive elements (metals) with oxygen (oxides) or more 

 generally with oxygen and hydrogen (hydroxides). Thus, silver 

 oxide, Ag 2 O, and sodium hydroxide, NaOH, are basic substances. 

 Other properties characteristic of bases are : 



1. When acted upon by acids, they form salts ; for instance, 

 when sodium hydroxide and nitric acid are brought together water 

 and the salt sodium nitrate are formed : 



NaOH + HN0 3 = H 2 O + NaNO 3 . 



2. They have (when soluble in water) an alkaline reaction, i. e., 

 they restore the color of organic substances when previously changed 

 by acids : for instance, that of litmus, from red to blue. 



3. They have (when soluble in water) the taste of lye, or an alka- 

 line taste. 



The term base was originally applied to the metallic oxides, because when 

 salts of the metals were highly heated they were decomposed, leaving a non- 

 volatile calx or ash, the oxide of the metal, while the acid radical of the salt 

 was driven off. Thus the metallic oxides were regarded as the base or stable 

 groundwork of the salts. In the present-day classification, metallic hydroxides 

 are called bases, but as the oxides bear such a close relationship to the hy- 

 droxides, in fact, many of them being converted into hydroxides in contact 

 with water, many authors also include metallic oxides in the class of bases. 

 The relationship between metallic oxides and hydroxides is well shown in the 

 case of quick-lime, calcium oxide. Nearly everyone is familiar with the 

 process of slaking lime by adding water to quick-lime. The action takes 

 place thus, CaO + H a O = Ca(OH) 2 . The slaked lime, Ca(OH) 2 , is a 



