THE MORE COMMON ELEMENTS AND COMPOUNDS 21 



made to react with a base, it first loses two molecules of water, as 

 already shown: 



H 6 SO 6 = H 2 SO 4 + 2 H 2 O. 



The compound H 2 SO 4 is common sulfuric acid, and it reacts 

 with calcium hydroxid in accordance with the following equation: 



-OH H| 



Ca< ! + ! >S0 4 = CaS0 4 + 2 H 2 O. 

 X |OH Hi' 



The dotted line shows how the two compounds are broken. The 

 two hydrogen atoms are broken off from the sulfuric acid and the 

 two hydroxyl groups are broken off from the calcium hydroxid, 

 and each hydrogen atom (H ) immediately joins hands with a 

 hydroxyl group ( OH) and thus are formed two molecules of 

 water, 2(H O H). The bivalent calcium atom (Ca=) also 

 grasps the two free hands of the radicle =SO 4 , and forms the 

 compound CaSO 4 , which is calcium sulfate. Thus, from a base 

 (alkali) and an acid, we have formed water and a neutral salt, 

 neither acid nor alkaline. 



This is a typical reaction between a hydroxid acid and a hydroxid 

 base, resulting in the formation of a salt. Many other salts may be 

 formed by similar reactions between basic hydroxids, oxids, or free 

 metals, on the one hand, and different kinds of acids, on the other 

 hand, which may contain oxygen in the form of hydroxid only, 

 or in both hydroxid and oxid form, or the acid may contain no 

 oxygen, but only hydrogen joined directly to some nonmetallic 

 element, as in hydrochloric acid (HC1) or hydrosulfuric acid (H 2 S). 



Salts are the most abundant and important compounds. The 

 earth's crust consists largely of insoluble mineral salts, called sili- 

 cates. These are, as a rule, very complex salts, one of the simplest 

 being common felspar, potassium aluminum silicate, KAlSi 3 O 8 . 

 This double salt is the principal source of potassium, one of the 

 essential elements of plant food. Common salt is sodium chlorid, 

 NaCl. Epsom salt is magnesium sulfate, MgSO 4 . Glauber salt is 

 sodium sulfate, Na 2 SO 4 . Even limestone is a kind of salt, calcium 

 carbonate, CaCO . 



For convenience, the formulas of salts containing oxygen are 



