514 



SMITH S INTERMEDIATE CHEMISTRY 



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 \y c 



pound over the cupric oxide, which oxidizes it to water and car- 

 bon dioxide. The first is absorbed in a weighed U-tube filled with 

 calcium chloride, and the second is caught in a weighed vessel con- 

 taining potassium hydroxide. From the increase in weight in 

 each case, the corresponding weights of hydrogen and carbon 

 (derived from the weighed portion of the organic compound) are 

 calculated. 



Tests for Copper. The blue color of cupric salts in dilute 

 solution is distinctive (p. 175). Hydrogen sulphide precipitates 

 cupric sulphide CuS (brownish-black) even from acid solutions 

 of cupric salts (see p. 463). 



More active metals, such as zinc or iron, displace copper from 

 solutions of its salts, so that a blade of a knife, for example, re- 

 ives instantly a red coating of copper when immersed in such a 

 solution: 



Fe + Cu++ -> Fe++ + Cu J . 



Coj^erJPlating. When platinum or carbon plates, connected 

 with a battery, are immersed in a solution of cupric sulphate, 

 copper is deposited on the negative plate (cathode). The S0 4 = 

 migrates towards the positive wire (anode) and there produces 

 oxygen and sulphuric acid: 



2SO 4 + 2H 2 O - 2H 2 S0 4 + O 2 | - 



If the anode is made of copper itself, 

 however, the S0 4 = migrates, but is 

 not discharged. Instead, copper goes 

 into solution (Fig. 117) as Cu++, in 

 amount equal to that deposited on the 

 other plate. Thus the quantity of 

 cupric sulphate in solution remains 

 unchanged, and the effect is, virtu- 

 ally, to transfer copper from the copper anode to the cath- 

 ode. 



