364 SMITH'S INTERMEDIATE CHEMISTRY 



bonate is separated by filtration, and the solution is concentrated 

 until crystals appear upon cooling. 



It is a white crystalline salt. It is added to the glass, used for 

 enamelling and glazing, to make it more fusible and easier to 

 spread in a thin layer. It is a preservative. Since it contains 

 but a small proportion of the metallic oxide (Na20,2B 2 3 ), it 

 combines with other metallic oxides when fused with them. For 

 this reason the powdered salt is sometimes sprinkled on tarnished 

 metallic surfaces which are to be soldered or brazed. The heat 

 of the bolt or blowpipe melts the borax, and the latter removes 

 the oxides and permits perfect running of the solder over the 

 surface. The borates thus formed are often colored, and the 

 colors afford a means of recognizing the metallic compound which 

 produced them. In chemical analyses a bead of borax, produced 

 by fusion on a platinum wire, is heated with a particle of the un- 

 known compound and its color then examined. The colors are 

 similar to those already described under colored glass (p. 362). 



When hydrochloric or nitric acid is added to a hot, concentrated 

 solution of borax, boric acid crystallizes out : 



NasB 4 7 + 2HC1 + 5H 2 O -4 2NaCl + 4H 3 B0 3 j . 



Boric acid may be recognized by the green tint which it confers 

 on the Bunsen or alcohol flame. 



Exercises. 1. Make an equation for the preparation of, (a) 

 lead silicate PbSiO 3 by fusion of litharge PbO and sand, (b) 

 potassium silicate K 2 SiO 3 . 



2. The inside surface of the bottle of sodium hydroxide solu- 

 tion beconies etched and dull. To what is this due? 



3. What is the valence of boron? 



4. Why is not all of the boric acid deposited from a hot solu- 

 tion containing it? 



5. Write an equation for the effect of heat upon borax. 



6. Why does the addition of borax render a glass more easily 

 fusible? 



