ZINC. 315 



ment 3, in about 200 c.c. of water, heat to boiling, and add slowly, while stir- 

 ring, concentrated solution of sodium carbonate until precipitation is complete. 

 After the precipitate has settled, pour off the liquid, and wash the former sev- 

 eral times with hot water by decantation. Then filter and wash the precipitate 

 again several times with hot water, drain, and dry. 



Heat some of the dried zinc carbonate gradually to redness in a porcelain 

 crucible with the cover on. What is formed? What color has it while hot? 

 When the crucible is cold, place the residue in a tube and add dilute acid. 

 Does any effervescence take place. Write reaction. Compare with experi- 

 ments 25 and 26. 



Zinc sulphate, Zinci sulphas, ZnSO 4 .7H 2 O = 285.4 ( White vit- 

 riol), is obtained by dissolving zinc in dilute sulphuric acid : 

 H 2 SO 4 + a:H 2 O + Zn = ZnSO 4 + xH 2 O -f 2H. 



If zinc be added to strong cold sulphuric acid, no decomposition 

 takes place, because there are no ions present, and an acid does not 

 exhibit acid properties unless ions *are formed, as explained in 

 Chapter 15. 



Dilute sulphuric acid scarcely acts on pure zinc, but addition of a few c.c. of 

 solution of cupric sulphate or platinic chloride causes brisk action. This is 

 due to the deposition of the copper or platinum on the zinc, thus forming an 

 electric couple, whereby solution of zinc is facilitated. 



Zinc sulphate forms small, colorless crystals, which are isomor- 

 phous with magnesium sulphate ; it is easily soluble in water. It is 

 so much like magnesium sulphate in appearance that it is sometimes 

 taken in mistake for the latter salt. The tests given below will dis- 

 tinguish between the two salts. 



Antidotes. Soluble zinc salts (sulphate, chloride) have a poisonous effect. 

 If the poison have not produced vomiting, this should be induced. Milk, 

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

 zinc forms an insoluble compound) should be given. 



Tests for zinc. 

 (Use a 5 per cent, solution of zinc sulphate.) 



1. Add to the solution some ammonium sulphide. A white precipi- 

 tate of zinc sulphide, ZnS, is produced, which is soluble in mineral 

 acids, but not in acetic acid. (Of the familiar metals, zinc is the only 

 one whose sulphide is white.) 



If the zinc salt is not pure, the sulphide may appear more or less 

 gray instead of white : 



ZnS0 4 + (NH 4 ) 2 S = (NH 4 ) 2 S0 4 + ZnS. 



