CHAPTER XV 

 Silicon and Its Compounds 



121. Occurrence. Silicon is found in nature in com- 

 bination with oxygen as silica, SiO 2 ; and with oxygen 

 and the metals as silicates. It is never found free but al- 

 ways in combination with other elements. Next to 

 oxygen it is the most abundant element found in nature. 

 In the form of silicates it is the basis of the composition 

 of nearly all rocks, and in the soil SiO 2 is found to the 

 extent of from 60 to 90 per cent. It is present in the 

 ash of plants and, to a slight extent, in animal bodies. 



122. Preparation and Properties. Silicon is separated 

 from its compounds with difficulty. By treatment in an 

 electric furnace, quartz or SiO 2 is reduced. 



Like carbon, silicon has crystalline and 

 amorphous forms. Pure quartz, SiO 2 , 

 and other forms of silicon, are insoluble 

 in nitric, hydrochloric, and sulfuric acids. 

 When acted upon by hydrofluoric acid, 

 silicon tetrafluorid, a gas is formed. 



SiO 2 + 4HF = SiF 4 + 2H 2 O. HF is Quart^stal. 



used extensively for the decomposition of silicates. 



123. Silicic Acid. When SiO 2 is fused with the hy- 

 droxids or carbonates of potassium or sodium, potassium 

 or sodium silicate is obtained : 



SiO 2 + K 2 C0 3 = K 2 SiO 3 + CO 2 . 

 Si0 2 + 4 KOH = K 4 SiO, + 2H 2 O. 

 The silicates of potassium and sodium are soluble in 



