426 PRINCIPLES OF CHEMISTRY 



hydroxide. 21 Silver oxalate and the halogen compounds of silver are 

 insoluble in water; hydrochloric acid and soluble chlorides give, 

 as already repeatedly observed, a white precipitate of silver chloride 

 in solutions of silver salts. Potassium iodide gives a yellowish 

 precipitate of silver iodide. Zinc separates all the silver in a metallic.' 

 form from solutions of silver salts. Many other metals and reducing 

 agents for example, organic substances also reduce silver from the 

 solutions of its salts. 



Silver nitrate, AgNO 3 , is known by the name of lunar caustic 

 (or lapis infernalis) ; it is obtained by dissolving metallic silver 

 in nitric acid. If the silver be impure, the resultant solution will 

 contain a mixture of the nitrates of copper and silver. If this mixture' 

 be evaporated to dry ness and the residue carefully fused at an\ 

 incipient red heat, all the cupric nitrate is decomposed, whilst the 

 greater part of the silver nitrate remains unchanged. On treating 

 the fused mass with water the latter is dissolved, whilst the cupric 

 oxide remains insoluble. If a certain amount of silver oxide be added 

 to the solution containing the nitrates of silver and copper, it displaces 

 all the cupric oxide. In this case it is of course not necessary to take" 

 pure silver oxide, but only to pour off some of the solution and to add 

 potassium hydroxide to one portion, and to mix the resultant pre- 

 cipitate of the .hydroxides, Cu(OH) 2 and AgOH, with the remaining 

 portion. 22 By these methods all the copper can be easily jremoved and 



21 If a solution of a silver salt be precipitated by sodium hydroxide, and aqueous 

 ammonia is added drop by drop until the precipitate is completely dissolved, the 

 liquid when evaporated deposits a violet mass of crystalline silver oxide. If moist silver 

 oxide be left in a strong solution of ainm6nia it gives a black mass, which easily decom- 

 poses with a loud explosion, especially when struck. This black substance is called 

 fulminating silver. Probably this is a compound like the other compounds of oxides 

 with ammonia, and in exploding the oxygen of the silver oxide forms water with the 

 hydrogen of the ammonia, which is naturally accompanied by the evolution of heat and 

 formation of g&seous nitrogen, or, as Raschig states, fulminating silver contains NAg 3 or 

 o&e of the amides (for instance, NHAg 2 =NH 3 + Ag 2 O H 2 O). Fulminating silver is also 

 formed when potassium hydroxide is added to a solution of silver nitrate in ammonia. 

 The dangerous explosions which are produced by, this compound render it heedful that, 

 great care be taken when salts of silver come into contact with ammonia and alkalis* 

 {see Chapter XVI., Note 26). 



' w So that we here encounter the following phenomena : copper displaces silver from 

 jbhe solutions of , its salts, and silver oxide displaces copper oxide from cupric salts. 

 Guided by the conceptions enunciated in Chapter XV., we can account for this in the 

 following manners The atomic volume of silver =10-8, and of copper = 7'2, of silver 

 oxide ='82, and of copper oxide = 18. A greater contraction has 'taken place in the for* 

 iqiaition of cupric oxide, CuO, than in the formation of silver oxide, Ag 2 O, since in the 

 former (187 = 6) the volume after combination with the oxygen has increased by very 

 little, whilst the volume of silver oxide is considerably greater than that of the metal it 

 contains [82 - (2 x ID'S) = 11*4]. Hence silver oxide is less compact than cupric oxide, 

 and is therefore less stable ; but, on the other hand, there are greater intervals 

 between.the atoms in silver oxide than in cupric oxide, and therefore silver oxide is able to 



