Physical Properties of Lead
|Lead is bluish grey, and when unoxidised has a bright, shining appearance, well seen in the crystals separated from a lead salt solution by zinc, in the formation of the so-called lead-tree. Lambert and Cullis have prepared pure lead by the method of Stas, and distilled it in a vacuum in a silica tube. The globules of metal obtained had a very brilliant lustre, resembling that of mercury or silver, the bluish tint usually associated with lead being entirely absent from them. The fresh surface of the metal rapidly becomes dull owing to oxidation, probably with the formation of the suboxide, Pb2O; and then presents the appearance ordinarily associated with lead. Lead is dimorphous, crystallising in the regular and monoclinic systems. Regular octahedra are produced when the molten metal solidifies, and are best obtained by allowing a crust to form and then pouring away the still liquid metal. From the state of vapour lead crystallises in combinations of the regular octahedron and cube. Plates, which appear to be monoclinic, are formed by the electrolysis of solutions of lead salts by feeble currents, but when strong currents are employed octahedral crystals separate. According to Elbs and Rixon, lead may be deposited during electrolysis, either in shining plates or as a sponge of microscopic needles, the spongy deposit being due to the presence of plumbic salt in the electrolyte. In the growth of the lead-tree, however, when a rod of zinc is immersed in a solution of lead acetate, the separation of plates is frequently followed by the appearance of a spongy deposit. A layer of spongy lead is formed on the cathode during the electrolysis of sulphuric acid with lead electrodes. |
The density of lead has been determined under various conditions. Lead that has been poured while molten into water has a density of 11.352 at 0° C. compared with water at 4° C., and this when the metal has been rolled becomes 11.358-11.365. When distilled in a vacuum lead has a density of 11.3415 at 20° C., which when the metal is submitted to a pressure of 10,000 atmospheres becomes 11.3470. Cohen and Helderman have found the density of pure lead at 25° C. compared with water at 4° C. to be 11.3299. At its melting-point (325° C.) the density of the solid metal is 11.005, and of the liquid metal 11.645, whence it appears that lead expands on solidifying. Lead melts at about 327° C.; Heycock and Neville found 327.6° C., Callendar, 327.7° C., and Holborn and Day, 326.9° C. When contained in a quartz tube in a cathode light vacuum lead begins to evaporate at about 1000° C.; under a short vapour column it boils at 1140°-1142° C., whilst under a pressure of 45 mm. its boiling-point is 1172-1173° C. Lead vapour is monatomic at 1870° C.; the metal is also monatomic in solution in mercury.
The spectrum of lead has been examined by Hartley, Hagenbach, and Konen, Lamprecht, and Klein.
The most intense lines in the spectrum of lead are as follow:
Arc: 2393.92, 2577.39, 2614.26, 2663.27, 2802.10, 2823.31, 2833.21, 2873.48, 3572.95, 3639.72, 3671.80, 3683.62, 3740.20, 4019.80, 4058.00, 5005.63.
Spark: 2802.10, 2833.12, 3573.03, 3639.72, 3683.64, 3740.28, 4058.00, 4245.42, 4387.11, 5609.00.
Lead is the softest of the common metals; it may be easily cut with a knife, and leaves a streak when rubbed upon paper. In Moh's scale of hardness (diamond - 10) lead is 1.5. Lead is tough and malleable, and so can be rolled out into thin foil; it cannot, however, be drawn into fine wire, and lead wire of the thickness of fine string can very easily be snapped. Pieces of lead foil can be welded together under a pressure of 2000 atmospheres.
Lead becomes harder and more brittle when alloyed with antimony, zinc, bismuth, or arsenic. The last-named metal is present in shot. The specific heat of lead has been the subject of numerous investigations. The following results were obtained by Naccari, at temperatures between 18° C. and 300° C.:
|18° C.||50° C.||100° C.||150° C.||200° C.||250° C.||300° C.|
It will be seen from these figures that lead obeys Dulong and Petit's law, since atomic weight × specific heat = 207×0.03 = 6.21.
Colloidal lead is obtained by reducing lead chloride solution with hydrazine in the cold.