450 SMITH'S INTERMEDIATE CHEMISTRY 



with zinc dust and baking it. The zinc protects the iron, primar- 

 ily because it excludes the air from the surface, and secondarily 

 because, even when the coating is broken, the zinc, being the 

 more active metal of the two (p. 54), is rusted first. Zinc is used 

 also in extracting silver from crude lead (p. 519), as the active 

 metal (anode) in batteries, and in several alloys (e.g. Babbitt's 

 metal, p. 326, brass, and German silver). In the laboratory 

 granulated zinc, made by pouring the melted metal in a thin 

 stream into water, and zinc dust (impure, contains ZnO), are the 

 forms commonly employed. 



Zinc Oxide ZnO and Hydroxide Zn(OH) 2 . The oxide is 



made by burning zinc vapor in air. It is yellow while hot, and 

 white when jflfcl. Mixed with oil, it is used as a paint (Chinese 

 white). It has less covering power than has white lead paint (4 

 coats of the former equal 3 of the latter), but it does not darken 



from exposure to hydrogen sulphide in the air (ZnS is white, P 



Kl M O-^'*' ^^* eft*-c*4-Ce 



black), 



Zinc hydroxide Zn(OH) 2 s^formed by precipitation. Both the 

 oxide and hydroxide are. weakly basic, and give salts with acids. 

 But with respect to ^strong bases they are weakly o^cidic, dissolving 

 for example in excess 'of sodium hydroxide, giving sodium zincate 

 Na.HZn0 2 . 



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Zinc Chloride ZnCl 2 . The chloride is formed in the action 

 of zinc or zinc oxide on hydrochloric acid. It is a white, deliques- 

 cent solid. Its aqueous solution gelatinizes cellulose and dissolves 

 it (p. 398), and thus is used in parchmentizing paper and in im- 

 pregnating wood to prevent decay. The aqueous solution is acid 

 (hydrolysis) and is used for cleaning the surface of metals before 

 soldering. Solder " runs " on a hot brass, copper, or lead surface, 

 provided the latter is clean, and adheres perfectly when cold. But 

 it does not dissolve oxides, or melt them, and therefore cannot 

 even reach the surface of the metal, much less adhere to it, if the 

 slightest tarnish is present. f t ^^^j^ ,__ 



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