302 METALS AND THEIR COMBINATIONS. 



Remarks to tests. In test 1 iron in the ferric state is too weakly 

 basic to form ferric sulphide, but in the ferrous state it is a stronger 

 base, so that the ferric sulphide breaks down at the moment of forma- 

 tion to ferrous sulphide and sulphur. If ferrous iron were as weakly 

 basic as aluminum and chromium, no precipitate of sulphide would 

 be obtained. 



In test 2 no precipitate is formed, because of the acid that would 

 be set free by the reaction. Ferric salts are easily reduced to ferrous 

 salts, and vice versa. H 2 S is a good reducing agent, and, when acting 

 as such, always gives a precipitate of milk of sulphur, which easily 

 passes through filter-paper and causes annoyance in the course of 

 qualitative analysis. 



In test 4 the weak basic character of ferric iron and the resem- 

 blance to aluminum and chromium is again shown. 



Tests 6, 7, 8, and 9 are not only delicate and decisive, but permit 

 iron in either state to be detected in the presence of the other. 



Ferrous compounds form the divalent ion Fe", which is pale-green, and 

 ferric compounds form the trivalent ion Fe*", which is nearly colorless. The 

 ionic reactions for the tests for iron are of the same form as those given under 

 the tests for calcium. In test 2, the reduction of ferric to ferrous salt by H 2 S 

 is expressed by the ionic equation : 



2Fe'" + 6C1' + 2H- + S" = 2Fe" + 2H* + 6C1' + S. 

 Each of the iron ions loses a charge of electricity and the sulphur ion loses 

 its two charges, which mutually neutralize each other, and elementary sulphur 

 is precipitated. The reduction with ammonium sulphide is represented by a 

 similar equation. 



The formation of ferric from ferrous chloride is expressed thus : 



Fe" + 2CP + Cl = Fe- + 3C1'. 



The iron ion assumes another positive charge, becoming trivalent ion, and the 

 chlorine atom assumes a negative charge, becoming chlorine ion. A similar 

 equation holds in the case of the sulphate. 



QUESTIONS Which metals belong to the " iron group," and what are their 

 general properties? How is iron found in nature, and what compounds are 

 used in its manufacture ? Describe the process for manufacturing iron on a 

 large scale, and state the difference between cast-iron, wrought-iron, steel, and 

 reduced iron. State the composition and mode of preparation of ferrous and 

 ferric hydroxides. What are their properties? Describe in words and chem- 

 ical symbols the process for making ferric chloride. What is tincture of chlo- 

 ride of iron? How are ferrous iodide and bromide made? State the proper- 

 ties of ferrous sulphate. Under what other names is it known, and how is it 

 made? What change takes place when soluble carbonates are added to soluble 

 ferrous and ferric salts? Mention agents by which ferrous compounds may be 

 converted into ferric compounds, and these into ferrous compounds. Explain 

 the chemical changes taking place. Mention tests for ferrous and ferric com- 

 pounds. 



