298 METALS AND THEIR COMBINATIONS. 



and free acid. Addition of acid, by preventing hydrolysis, renders the solu- 

 tions colorless or nearly so. Hydrolysis is increased by heating the solutions, 

 hence hot ferric solutions have a deeper color than cold ones. 



Ferrous salts are much less hydrolyzed than ferric salts, as ferrous iron is a 

 stronger base than ferric iron. They also are not as acid to litmus as the 

 ferric salts. 



Dialyzed iron is an aqueous solution of about 5 per cent, of ferric hydrox- 

 ide with some ferric chloride. It is made by slowly adding ammonium hy- 

 droxide to a solution of ferric chloride as long as the precipitate of ferric 

 hydroxide formed is redissolved in the ferric chloride solution, on shaking 

 violently. The clear solution thus obtained is placed in a dialyzer floating in 

 water which latter is renewed every day until it shows no reaction with silver 

 nitrate. The ammonium chloride passes through the membrane of the dialyzer 

 into the water, while all iroir as hydroxide with some chloride, is left in 

 solution. 



The combination of an oxide or hydroxide with a normal salt is called 

 usually a basic salt or oxy-salt ; dialyzed iron is a highly basic oxychloride 

 of iron. 



Ferrous iodide, Fel^ and Ferrous bromide, FeBr 2 , may both be 

 obtained by the action of iodine and bromine, respectively, on iron filings, 

 when combination takes place. Both salts are unstable, absorbing oxygen 

 from the air very readily. 



Experiment 31. Cover some fine iron wire with water, heat gently, and add 

 iodine in fragments as long as the red color of iodine disappears. Notice that 

 the iron is dissolved gradually, the result of the reaction being the formation 

 of a pale-green solution of ferrous iodide. 



Ferrous sulphide, FeS. Easily obtained as a black, brittle mass, 

 by heating iron filings with sulphur, when the elements combine. It 

 is used chiefly for liberating hydrogen sulphide, by the addition of 

 sulphuric acid. Iron combines with sulphur in several proportions; 

 some of these iron sulphides are found in nature. 



Ferrous sulphate, Ferri sulphas, FeSO 4 .7H 2 O = 276.O1 (Sul- 

 phate of iron, Green vitriol, Copperas). Obtained by dissolving iron 

 in dilute sulphuric acid, evaporating, and crystallizing : 

 Fe + H 2 S0 4 = 2H + FeSO 4 . 



Also obtained as a by-product in some branches of chemical indus- 

 try, and by heap-roasting of the native iron sulphide : 



FeS 2 + 60 = FeS0 4 + SO 2 . 



Ferrous sulphate crystallizes in large, bluish-green prisms ; it is 

 soluble in water, insoluble in alcohol. Exposed to the air, it loses 

 water of crystallization and absorbs oxygen. 



