324 METALS AND THEIR COMBINATIONS. 



Cupric compounds are more numerous, as they embrace the salts of 

 oxygen acids as well as salts of some of the halogen acids. The 

 anhydrous salts are usually white or yellow, but the solutions as well as 

 the hydrated crystals are usually blue or greenish blue. The cupric 

 compounds are more important and familiar than the cuprous salts, 

 and those most frequently employed are the sulphate, acetate, and 

 oxide. The valence of copper in cupric compounds is 2. Some facts 

 indicate that in the cuprous compounds the valence of copper is 1, 

 while others indicate that it is 2. Some writers assign the valence of 

 2 to copper in all its compounds, and use double formulas for the 

 cuprous salts to account for the apparent univalence of copper, thus, 

 Cu 2 Cl 2 or Cl-Cu Cu-Cl, assuming that two atoms of copper are 

 united by one bond. The behavior of the cuprous salts can very 

 readily be represented by the simple formulas and univalence of copper, 

 and there is no need for using double formulas. 



Cupric oxide, CuO (Black oxide or monoxide of copper). Heated 

 to redness, copper becomes covered with a black scale, which is cupric 

 oxide ; it is obtained also by heating cupric nitrate or carbonate, both 

 compounds being decomposed with formation of the oxide ; finally, 

 it may be made by adding sodium or potassium hydroxide to the 

 solution of a cupric salt, when a bulky, pale-blue precipitate of cupric 

 hydroxide, Cu(OH) 2 , is formed, which, upon boiling, is decomposed into 

 water and cupric oxide, a heavy dark-brown powder (Plate III., 2) : 



CuS0 4 + 2KOH = K 2 SO 4 + Cu(OH) 2 ; 

 Cu(OH) 2 = H 2 O + CuO. 



Cuprous oxide, Cu 2 O ( Red oxide or suboxide of copper). When 

 cupric oxide is heated with metallic copper, charcoal, or organic 

 matter, the cupric oxide is decomposed, and cuprous oxide is formed. 

 (Excess of carbon or organic matter reduces the oxide to metallic 

 copper.) 



CuO -f Cu = Cu 2 O; 

 2CuO + C = Cu 2 O + CO. 



Some organic substances, especially grape-sugar, decomposes alkaline 

 solutions of cupric sulphate with precipitation of cuprous oxide, which 

 is a red, insoluble powder. 



Experiment 40. To 5 c.c. of a 5 per cent, solution of copper sulphate add 

 about 20 c.c. of the reagent solution of sodium hydroxide. Note the blue pre- 

 cipitate of copper hydroxide, Cu(OH) 2 . Add to the mixture about 2 grammes 

 of Rochelle salt (sodium potassium tartrate) and shake. The precipitate dis- 



