360 SMITH'S INTERMEDIATE CHEMISTRY 



Diatomaceous earth (Tripoli powder) is used in making polishing 

 powders and for removing coloring matters from oils. 



Chemical Properties. Silicon dioxide (sand) is not acted 

 upon by acids, with the exception of hydrofluoric acid, which 

 gives silicon tetrafluoride SiF 4 and water: 



Si0 2 + 4HF -> SiF 4 T + 2H 2 0. 



When silicon dioxide is fused with sodium carbonate, carbon 

 dioxide gas is liberated, and sodium silicate Na 2 Si0 3 is formed: 



Si0 2 -I- Na^COs - C0 2 1 + 



The resulting salt is very soluble and a concentrated aqueous solu- 

 tion is sold under the name of water-glass or soluble glass. This 

 material is used as a filler in cheap soaps, as an ingredient in 

 artificial stone, a coating to render wood or cloth fireproof, and a 

 cement for uniting glass or porcelain. Eggs are preserved by 

 being submerged in a solution of this salt. 



When an acid is added to sodium silicate solution, silicic acid 

 H 4 Si04 or H 2 SiO 3 (gelatinous, the degree of hydration varies with 

 the conditions) is precipitated. If this acid is heated strongly, 

 silica Si0 2 remains as a powder. 



Incompletely dehydrated silicic acid, containing 5-7 per cent 

 of water (silica gel), is employed as an adsorbent material for 

 recovering valuable vapors (such as sulphur dioxide, oxides of 

 nitrogen, and volatile organic solvents) from the issuing gases 

 in many large-scale industrial processes. The adsorbed vapors 

 are given up on heating, and the gel is ready for renewed use. 

 Silica gel is also of service in deodorizing petroleum oils, and as a 

 catalyst (see p. 262) . 



Silicon Tetrachloride SiCl 4 . This compound is made by 

 direct union of the free elements. It is more conveniently pre- 

 pared by passing chlorine over a strongly heated mixture of silicon 



