CLASSES OF SUBSTANCES 185 



for example, mercury is a liquid ; sodium and potassium 

 float upon water and are also soft. Thus it is difficult 

 to find any common property by which to define a 

 metal, and in this study we must be content to learn 

 some of the important metallic elements, together with 

 their general behavior. 



By far the greater number of elements are metals. 

 Some of these are very common on earth, while others 

 are very rare. A few metals (e.g. iron, copper, and 

 zinc) are of much importance in the life of man; but 

 there are also several whose existence is never realized 

 by us, and whose very names are never heard except 

 among scientists. 



A few metals are sometimes found free in the earth, 

 though most of them occur only in compounds with 

 other elements. Pure metals are obtained by breaking 

 up the salts or the ores in which they occur. The fol- 

 lowing metallic elements are familiar: Al, Bi, Ca, Cu, 

 Au, Fe, Pb, Hg, Ni, Pt, K, Ag, Na, Sn, Zn ( 208). Of 

 these, Ca, Na, and K are not common in a free (not 

 combined) condition. 



213. Salts. The salts form a large and important 

 group of substances. Many different salts may be 

 formed by the action of metals upon acids, or of bases 

 upon acids. In either case, a salt is formed when the 

 hydrogen of an acid is set free and some metal taken on 

 in its place. 



Experiment 116. To a little hydrochloric acid (HC1) in a 

 test tube add a piece of zinc (Zn). Note the action. Bubbles 

 show that a gas is given off ; this is hydrogen (H). When the 

 action ceases, boil the liquid to dryness. Describe the substance 



