269. Properties.—The varying properties of pig-iron, steel, and wrought-iron are due in part to the proportion of C and of other elements present, either as mixtures or as compounds, and in part to other causes not well understood. Wrought-iron is fibrous, as though composed of fine wires, and hence is ductile, malleable, tough, and soft, and cannot be hardened or tempered, but it is easily welded. Pig-iron is crystalline, and so is not ductile or malleable; it is hard and brittle, and cannot be welded. On account of its low melting-point it is generally employed for castings. Steel is crystalline in structure, and when suddenly cooled from red heat by plunging into cold water, becomes hard and brittle. The tempering can be varied by afterwards heating to any required degree, indicated by the color of the oxide formed on the exterior. The higher temperatures give the softer steel.
270. Salts of Iron.—Examine FeSO4, FeS, FeS2.
Fe has a valence of 2 or 4. This gives rise to two kinds of salts, ferrous and ferric, as in FeCl2 and Fe2Cl6 The valence of Fe in ferric salts is 4. Ferrous sulphate is FeSO4; ferric sulphate, Fe2(SO4)3. Write the symbols for ferrous and ferric hydrate; for the oxides; for the nitrates. Write the graphic symbols for each.
271. Colors.—The characteristic color of ferrous salts is green, as in FeSO4. These salts give the green color to the chlorophyll in leaves and grass, and bottle glass owes its green color to ferrous silicate. Ferric salts are a brownish red, as shown in hematite and limonite, and in some bottles. Red sandstone, and most soils and earths, are illustrations of this coloring action. The blood of vertebrates owes its color to ferric salts. Bricks are made from a greenish blue clay in which iron exists in the ferrous state. On being heated, ferrous salts are oxidized to ferric, and their color is changed to red. Iron rust is hydrated ferric oxide, Fe2O3 and Fe2(OH)6.
272. Change of Valence.
Experiment 122.—Dissolve 2 g. of iron filings in diluted HCl. Filter or pour off the clear liquid, divide it into two parts, and add NH4OH to one part till a ppt. occurs. Notice the greenish color of Fe(OH)2. Oxidize the other part by adding a few drops of HNO3 and boiling a minute. Now add NH4OH, and observe the reddish color of the ppt., Fe2(OH)6.
Solutions of ferrous salts will gradually change to ferric, if allowed to stand, thus showing the greater stability of the latter. In changing from FeCl2 to Fe2Cl6 oxidation does not consist in adding O, but in increasing the negative element or radical. This is possible only by changing the valence of Fe from 2 to 4. Hence oxidation, in its larger sense, means increasing the valence of the positive element. To oxidize FeSO4 is to make it Fe2(SO4)3, changing the valence of Fe as before. Reduction or deoxidation diminishes the valence of the positive element. Illustrate this by the same iron salts. Illustrate it by PbO and Pb02; AuCl and AuCl3; Sb2S3 and Sb2S5. In this sense define an oxidizing agent. A reducing agent.
273. Ferrous Sulphate.
Experiment 123.—Dissolve a few iron filings in dilute H2SO4, and slowly evaporate for a few minutes. Write the equation.
Ferrous sulphate, green vitriol, or copperas, FeSO4 + 7 H2O, is the source of what acid? See page 66. It is also one of the ingredients in many writing inks. On being heated, or exposed to the air, it loses its water of crystallization and becomes a white powder. It is prepared as above, or by oxidizing moistened FeS2 by exposure to the air.