ZINC. 317 



2. Hydrogen sulphide passed into the solution gives a partial pre- 

 cipitate of zinc sulphide because of the solvent action of the acid 

 liberated. In the presence of sodium acetate, however, the precipi- 

 tate is complete because of the liberation of acetic acid, in which the 

 sulphide is insoluble : 



ZnS0 4 -I- 2Na(C 2 H 3 2 ) + H 2 S = ZnS + Na 2 SO 4 + 2H.C 2 H 3 2 . 



3. Addition of caustic alkali or ammonia water gives a white pre- 

 cipitate of zinc hydroxide, Zn(OH) 2 . It is soluble in excess of the 

 alkali, forming zincates : 



Zn(OH) 2 + 2NaOH = Zn(ONa) 2 + 2H 2 0. 



The hydroxide is also soluble in excess of ammonia water, forming 

 a complex compound, Zn(NH 3 ) 4 .(OH) 2 . In this respect zinc differs 

 from magnesium. The ions of this compound are Zn(NH 3 ) 4 " and 

 2(OH)'. 



4. Addition of a solution of acarbftnate or phosphate, gives a white 

 precipitate of zinc carbonate or phosphate : 



ZnSO 4 -f Na 2 HPO 4 = ZnHPO 4 + Na 2 S0 4 . 



Zinc carbonate is soluble in excess of ammonium carbonate. 



5. Solution of potassium ferrocyanide gives a white precipitate of zinc 

 ferrocyanide. (Distinction from magnesium and aluminum, which give 

 no precipitate.) The precipitate is Zn 2 Fe(CN) 6 , and is difficultly 

 soluble in hydrochloric acid. 



Tests 1, 3, and 5 together are conclusive for zinc salts. Practically 

 all the compounds of zinc are colorless. The oxide, carbonate, phos- 

 phate, ferrocyanide, and sulphide are insoluble in water; the chloride, 

 bromide, iodide, nitrate, sulphate, and acetate are soluble in water. 

 These are the common salts. The soluble zinc salts are hydrolyzed 

 somewhat in water, and therefore show an acid reaction. This explains 

 the solvent action of a zinc chloride solution when used on metal sur- 

 faces in soldering. The coat of metallic oxide is thus removed. 



Zinc forms the divalent ion Zn", which unites with acid radicals to form 

 the zinc salts. The ionic equations for the above tests are of the same form as 

 those given under the tests for calcium. Zn(OH) 2 has weak basic properties, 

 and still weaker acid properties. Like aluminum and chromium hydroxides, 

 it is slightly soluble and ionizes in two ways, thus : 



Zn(OH) 2 ^ Zn" 4 2(OH)'. 

 With acids, zinc salts are formed by the union of (OH) 7 and H* ions to form water. 



Also, Zn(OH) 2 ; Zn0 2 " 4- 2H', 



and with considerable excess of alkali the hydroxide dissolves to form zincates 



