408 PRINCIPLES OF CHEMISTRY 



When copper is oxidised with a considerable quantity of oxygen at 

 a high temperature, or at the ordinary temperature in the presence of 

 acids, and also when it decomposes acids, converting them into lower 

 grades of oxidation (for example, when submitted to the action of 

 nitric and sulphuric acids), it forms cupric oxide, CuO, or, in the 

 presence of acids, cupric salts. Copper rust, or that black mass which 

 forms on the surface of copper when it is calcined, consists of cupric 

 oxide. The coating of the oxidised copper is very easily separated 

 from the metallic copper, because it is brittle and very easily peels off, 

 when it is struck or immersed in water. Many copper salts (for 



nium chloride. By the action of a certain excess of ammonia on a hydrochloric acid 

 solution of cuprous chloride, very well formed crystals, having the composition 

 CuCl,NH3,H 3 O, are obtained. Cuprous chloride is not only soluble in ammonia and 

 hydrochloric acid, but it also dissolves in solutions of certain other salts for example, 

 in sodium chloride, potassium chloride, sodium thiosulphate, and certain others. All 

 the solutions of cuprous chloride act in many cases as very powerful deoxidising 

 substances; for example, it is easy, by means of these solutions, to precipitate 

 gold from its solutions in a metallic form, according to the equation AuCls + SCuCl 

 = Au + 8CuCl 2 . 



Among the other compounds corresponding with cuprous oxide, cuprous iodide, Cul, 

 is worthy of remark. It is a colourless substance which is insoluble in water and 

 sparingly soluble in ammonia (like silver iodide), but capable of absorbing it, and in this 

 respect it resembles cuprous chloride. It is remarkable from the fact that it is exceed- 

 ingly easily formed from the corresponding cupric compound Culs- A solution of cuprio 

 iodide easily decomposes into iodine and cuprous iodide, even at the ordinary tempera- 

 ture, whilst cupric chloride only suffers a similar change on ignition. If a solution of a' 

 cupric salt be mixed with a solution of potassium iodide the cupric iodide formed imme- 

 diately decomposes into free iodine and cuprous iodide, which separates out as a precipi- 

 tate. In this case the cupric salt acts in an oxidising manner, like, for example, nitrous 

 acid, ozone, and other substances which liberate iodine from iodides, but with this differ- 

 ence, that it only liberates half, whilst they set free the whole of the iodine from potas- 

 sium iodide : 2KI + CuCl 2 = 2KC1 + Cul + 1. 



It must also be remarked that cuprous oxide, when treated with hydrofluoric acid, 

 gives an insoluble cuprous fluoride, CuF. Cuprous cyanide is also insoluble in water, 

 and is obtained by the addition of hydrocyanic acid to a solution of capric chloride 

 saturated with sulphurous anhydride. This cuprous cyanide, like silver cyanide, gives 

 a double soluble salt with potassium cyanide. The double cyanide of copper and 

 potassium is tolerably stable in the air, and enters into double decompositions with 

 various other salts, like those double cyanides of iron with which we are already 

 acquainted. 



Copper hydride, CuH, also belongs to the number of the cuprous compounds. It 

 was obtained by Wiirtz by mixing a hot (70) solution of cupric sulphate with a solution 

 of hypophosphorous acid, H 3 PO 2 . The addition of the reducing hypophosphorous acid 

 must be stopped when a brown precipitate makes its appearance, and when gas begins 

 to be evolved. 'The brown precipitate is the hydrated cuprous hydride. When gently 

 heated it disengages hydrogen ; it gives cuprous oxide when exposed to the air, burns 

 in a stream of chlorine, and liberates hydrogen with hydrochloric acid: CuH + HCl 

 = CuCl + H 2 . Zinc, silver, mercury, lead, and many other heavy metals do not form 

 such a compound with hydrogen, neither tinder these circumstances nor under the action 

 of hydrogen at the moment of the decomposition of salts by a galvanic current. The 

 greatest resembance is seen between cuprous hydride and the hydrogen compounds of 

 potassium, sodium, Pd, Ca, and Ba. 



