ZINC. 183 



white of egg, or, still better, some substance containing tannic acid (with which 

 zinc forms an insoluble compound) should be given. 



Analytical reactions. 

 (Zinc sulphate, ZnSO 4 , may be used.) 



1. Add to solution of a zinc salt ammonium sulphide; a white 

 precipitate of zinc sulphide, ZnS, is produced. (Zinc sulphide is the 

 only white insoluble sulphide.) 



ZnSO 4 + (NH 4 ) 2 S = (NH 4 ) 2 SO 4 + ZnS. 



2. From neutral zinc solutions, or from solutions containing free 

 acetic acid, hydrogen sulphide precipitates white zinc sulphide. 



3. Add ammonium, sodium, or potassium hydroxide : a white pre- 

 cipitate of zinc hydroxide, Zn(OH) 2 , is produced, soluble in excess of 

 the reagent, with the formation of zincates, such as Zn (OK) 2 . 



4. Soluble carbonates and phosphates give white precipitates in 

 neutral solutions of zinc. 



5. Potassium ferrocyanide gives a white precipitate of zinc ferro- 

 cyanide. (This test may be used to distinguish compounds of zinc 

 from those of magnesium or aluminum.) 



6. Zinc is the only heavy metal whose compounds are all colorless. 

 The oxide, carbonate, phosphate, and ferrocyanide are insoluble ; the 

 chloride, nitrate, and sulphate soluble. 



Cadmium, Cd = 111.5. Found in nature associated (though in very small 

 quantities) with the various ores of zinc, with which metal it has in common a 

 number of physical and chemical properties. Cadmium differs from zinc by 

 forming a yellow sulphide (with hydrosulphuric acid), insoluble in diluted acids. 

 Oadmium and its compounds are of little interest here; the yellow sulphide is 

 used as a pigment, the sulphate and iodide sometimes for medicinal purposes. 



QUESTIONS. 261. How is zinc found in nature, and by what process is it 

 obtained ? 262. Mention the properties of metallic zinc, and what is it used 

 for? 263. Mention two processes for making zinc oxide. 264. How does heat 

 act on zinc oxide ? 265. Show by chemical symbols the action of hydrochloric 

 and sulphuric acids on zinc. 266. State the properties of chloride and of 

 sulphate of zinc ? 267. What is white vitriol ? 268. Explain the formation of 

 precipitated zinc carbonate, and state its composition. 269. Mention tests for 

 zinc compounds. 270. How many pounds of crystallized zinc sulphate may 

 be obtained from 21.7 pounds of metallic zinc ? 



