ELEMENTS 199 



230. Sodium and Potassium. The solid metallic ele- 

 ments Na (sodium) and K (potassium) are not found free 

 in nature. Their salts, however, are very common and 

 important. The elements may be separated from some 

 of their salts. Neither is common outside of labora- 

 tories, and no great use is made of them. They are 

 soft, waxy metals, lighter than water. Na acts upon 

 water to decompose it, and K does the same, but more 

 strongly. 



Experiment 127. Cut a small piece each of Na and K (the 

 size of a small pea). Throw the Na on some water in a dish, 

 being careful then to keep away from it. Next do the same with 

 the K. What difference in these two cases ? Try to explain how 

 this difference proves that K acts upon water more strongly than 

 Na (see 206). 



Of the salts of Na, NaCl is common and important ; 

 also Na 2 SO 4 (sodium sulphate) and NaNO 3 . Potassium 

 carbonate, K 2 CO 3 , occurs in the earth, is absorbed by 

 plants, and forms a part of wood ashes ; KNO 3 (salt- 

 peter) is an important salt of K. Na and K form strong 

 bases or alkalis, NaOH and KOH. 



231. Calcium. Calcium (Ca) is a solid metallic ele- 

 ment ; like Na and K, it is common only in compounds 

 with other elements. Some of its compounds, however, 

 are important and are found in large quantities. CaCO 3 , 

 calcium carbonate, occurs widely in the earth ; in differ- 

 ent forms it is called limestone, marble, or chalk. CaSO 4 , 

 sometimes called gypsum, also occurs in the earth; when 

 heated it forms a white powder, plaster of Paris. 

 The oxide of calcium, CaO, is called lime; it is used 



