22 SCIENCE AND SOIL 



written with the oxygen in one group, as Na^SO^ but it may well 

 be remembered that oxygen is regularly the linking element be- 

 tween the metal and the nonmetal, and the structure of the mole- 

 cule is better shown if written thus: 



A few more equations for typical reactions will show the various 

 ways by which salts may be formed. Other similar acids and bases 

 give similar products: 



Ca(OH) 2 + H 2 SO 4 = CaSO 4 + 2 H 2 O. 

 KOH + HNO 3 = KNO 3 + H 2 O. 



These equations represent, first, the reaction between the cal- 

 cium hydroxid and sulfuric acid with the formation of calcium 

 sulfate (gypsum, or land plaster), and, second, the reaction be- 

 tween potassium hydroxid and nitric acid with the formation of 

 potassium nitrate (saltpeter). 



In place of a hydroxid base, we may use an oxid of some metal, 

 thus: 



CaO + H 2 SO 4 = CaSO 4 4- H 2 O. 



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



K 2 O + H 2 SO 4 = K 2 SO 4 + H 2 O. 



The salts formed are calcium sulfate, calcium nitrate, and potas- 

 sium sulfate, and in each reaction only one molecule of water is 

 formed, when the oxid base is used, in place of two molecules of 

 water from the use of a bivalent hydroxid base. 



The base may be not only in the form of an oxid or hydroxid of 

 the metal, but we may use the free metal itself as a base, thus : 



Fe + H 2 SO 4 = FeSO 4 + H 2 . 



Mg + 2 HN0 3 = Mg(N0 3 ) 2 + H 2 . 



In these reactions the hydrogen atoms are displaced by the metal 

 and liberated as a gas. 



While most acids contain oxygen either as a hydroxid (C1OH) 

 or a hydroxid oxid (OC1OH orO 2 C!OH), a few acids, with the prefix 

 hydro-, contain no oxygen; as, for example, hydrochloric acid 



