410 PRINCIPLES OF CHEMISTRY 



potassium or sodium hydroxide does nob dissolve cupric hydroxide. 

 A hot solution gives a black precipitate of the anhydrous oxide 

 instead of the blue precipitate, and the precipitate of the hydroxide 

 of copper becomes granular, and turns black when the solution 

 is heated. This is due to the fact that the blue hydroxide is 

 exceedingly unstable, and when slightly heated it loses the elements 

 of water and gives the black anhydrous cupric oxide : CuH 2 2 

 = CuO + H 2 O. 



Cupric oxide fuses at a strqng heat, and on cooling forms a heavy 

 crystalline mass, which is black, opaque, and somewhat tenacious. It 

 is a feebly energetic base, so that not only do the oxides of the metals 

 of the alkalis and alkaline earths displace it from its compounds, but 

 even such oxides as those of lead and silver precipitate it from solutions, 

 which is partially dueto these oxides beingsoluble, although butslightly 

 so, in water. However, cupric oxide, and especially the hydroxide, 

 easily combines with even the least energetic acids, and does not give 

 any compounds with bases ; but, on the other hand, it easily forms 

 basic salts, 9 bis and in this respect outstrips magnesium and recalls the 



aqueous ammonia. Copper nitride is very stable, and is insoluble ; it has the composi- 

 tion Cu 3 N (i.e. the copper is monatomic here as in Cu 2 O), and is an amorphous green 

 powder, which is decomposed when strongly ignited, and gives cuprous chloride and 

 ammonium chloride when treated with hydrochloric acid. Like the other nitrides, copper' 

 nitride, Cu 3 N, has scarcely been investigated.. Granger (1892), by heating copper in the 

 vapour of phosphorus, obtained hexagonal prisms of 'Cu 5 P, which passed into_Cu 6 P 

 (previously obtained by Abel), when heated in nitrogen. Arsenic is easily absorbed by 

 copper, and its presence (like P), even in small quantities, has a great influence upon 

 the properties of copper for instance, pure copper wire 1 sq. mm. in section breaks 

 under a load of 85 kilos, while, the presence of 0'22 p.c. of arsenic raises the breaking 

 load to 42 kilos. 



9 bis As a comparatively feeble base, oxide of copper easily forms both basic and 

 double salts. As an instance we may mention the. double sails composed of th 

 dichloride CuCl 2 ,2H 2 and potassium chloride. The, double salt CuK 2 Cl4,2H 2 O 

 crystallises from solutions in blue plates, but when heated alone or with substances 

 taking up water easily gives brown needles CuKCl 3 and at the same time KC1, and this 

 reaction is reversible at 92 ' as Meyerhoffer (1889) showed (i.e. above 92 the simpler 

 double salt is formed and below 92 the more complex salt). With an excess of the 

 copper salt, KC1 gives another double salt, Cu 2 KCl 5 ,4H 2 O, the_transition temperature of 

 which is 65. The instances of equilibria which are encountered in such complex 

 relations (see Chapter XIV-., Note 25, .astrakhanite, and Chapter XXII., Note 23)- are 

 embraced by the law of phases given by Gibbs (Transactions of the Connecticut 

 Academy of Sciences, 1875-1878, in J. Willard Gibbs' memoir ' On the equilibrium of 

 heterogeneous substances : ' and in" a clearer and more accessible form in H. W, 

 Bakhuis Roozeboom's papers, Rec. trav. chim., Vol. VI., and in W. Meyerhoffer's memoir 

 Die Phasenregel und ihre Anwendungen t 1893, to which sources we refer those desiring 

 fuller information respecting this law). Gibbs calls ' bodies ' substances (simple or com- 

 pound) capable of forming homogeneous complexes (for instance, solutions or inter- 

 combinations) of a varied composition ; &phase a mechanically separable portion of such 

 bodies or of their homogeneous complexes (for instance, a vapour, liquid or precipitated 

 . solid), perfect equilibrium-^-mdh a state of bodies and t>f their complexes as. Is 



