194 METALS AND THEIR COMBINATIONS. 



While many of the non-metallic elements have long been known to exist in 

 allotropic forms, none of the metals had been obtained in such a condition 

 until quite recently, when it was shown that silver is capable of assuming a 

 number of allotropic modifications. These are obtained chiefly by precipi- 

 tating silver from solutions by different reducing agents. While normal silver 

 is white, the allotropic forms have distinct colors blue, bluish-green, red, pur- 

 ple, yellow and differ also in many other respects. Thus they are converted 

 into silver chloride by highly diluted hydrochloric acid, which does not act on 

 common silver; they are soluble in ammonia water, and act as reducing agents 

 upon a number of substances, such as permanganates, ferricyanides, etc. Allo- 

 tropic silver can be converted into the common form by different forms of 

 energy for instance, by heat, electricity, and the action of strong acids. 



Silver is too soft for use as coin or silverware, and, therefore, is 

 alloyed with from 5 to 25 per cent, of copper, which causes it to be- 

 come harder, and consequently gives it more resistance to the wear 

 and tear by friction. 



Pure silver may be obtained by dissolving silver coin in nitric acid, 

 when a blue solution, containing the nitrates of copper and silver, is 

 formed. By the addition of sodium chloride to the solution a white 

 curdy precipitate of silver chloride, AgCl, forms, while cupric nitrate 

 remains in solution. The silver chloride is washed, dried, mixed 

 with sodium carbonate, and heated in a crucible, when sodium chlo- 

 ride is formed, carbon dioxide escapes, and a button of silver is found 

 at the bottom of the crucible : 



2AgCl + Na,C0 8 = 2NaCl + CO 2 + 2Ag + O. 



Experiment 34. Dissolve a small silver coin in nitric acid, dilute with water, 

 and precipitate the clear liquid with an excess of solution of sodium chloride. 

 The washed precipitate of silver chloride may be treated with sodium carbon- 

 ate, as stated above, or may be converted into metallic silver by the following 

 method : Place the dry chloride in a small porcelain crucible and apply a 

 gentle heat until the chloride has fused ; when cold, place a piece of sheet 

 zinc upon the chloride, cover with water, to which a few drops of sulphuric 

 acid have been added, and set aside for a day, when the silver chloride will be 

 found to have been decomposed with liberation of metallic silver and forma- 

 tion of zinc chloride : 



2AgCl + Zn = ZnCl 2 + 2Ag. 



Wash the spongy silver with dilute sulphuric acid and then with water. 

 Use this silver for making silver nitrate by dissolving it in nitric acid, and 

 evaporation of the solution to dryness. Use this solution for silver reactions. 



Silver nitrate, Argenti nitras, Ag-NO 3 = 169.7. Pure silver is 

 dissolved in nitric acid : 



3Ag + 4HN0 3 = NO + 2H 2 O + 3AgNO 8 . 



