70 CHEMISTRY. 



Acids containing one atom of hydrogen are said to be mono-basic ; when 

 they contain two or three atoms of hydrogen, they are called di-basic or tri- 

 basic. Thus nitric acid, HN0 3 , is mono-basic, and sulphuric acid, H 2 S0 4 , 

 is di-basic. 



80. Bases and Salts. You have seen that blue litmus is 

 turned red by acids ; now there is another class of bodies 

 which turns reddened litmus to blue again. These bodies, 

 chemically opposed to the acids, are called bases. They 

 are either oxides or hydrates of the metals. Thus sodium 

 oxide, Na 2 O, and potassium hydrate, KHO, and calcium hy- 

 drate, CaH 2 O 2 , are bases. The soluble hydrates are called 

 alkalies, and possess strong caustic properties. Now when 

 the hydrogen of an acid is exchanged for a metal, or when 

 the acids act upon these bases, a third class of bodies is pro- 

 duced, called salts. Thus the hydrogen in nitric acid may 

 be replaced by silver, forming silver nitrate, which is a salt. 

 When the hydrates or the oxides are acted upon by acids, 

 water is formed at the same time with the salts, as shown 

 in the two examples below : 



Potassium hydrate, Nitric Potassium nitrate, Water. 



A base. Acid. A salt. 



KHO + HN0 3 = KNO 3 + II 3 



Calcium oxide, Nitric Calcium nitrate, Water. 



A base. Acid. A salt. 



CaO + 2HNO 3 = Ca(NO 3 ) a + H 2 O 



Having in the second example taken a dyad metal and an acid contain- 

 ing only one atom of hydrogen, two molecules of the acid are necessary to 

 complete the equation and form the salt. 



When an acid acts upon a base to form a salt, a remark- 

 able change in the properties of both the acid and the base 

 takes place ; the acid loses its corrosive, acid properties, 

 and the base loses its alkaline and caustic nature, the re- 

 sulting body being neutral. Neither reddened nor blue 

 litmus are affected by neutral salts. 



