Physical Properties of Tin
|Tin exists in three allotropic modifications. The form stable at ordinary atmospheric temperature (above 18° C.) is in tetragonal crystals, which at low temperatures pass more or less rapidly.into "grey tin," and at temperatures somewhat below the melting-point undergo transition into rhombic crystals. The following properties are those of tetragonal tin: |
Tin is a white, highly lustrous metal, having a density at 16° C. of 7.301 or 7.3137 after heating to near its melting-point. The density of the freshly fused metal is 7.287 at 15° C. Tin expands in melting; the density of the solid metal at 226.5° C. is 7.1835, and that of the liquid metal at the same temperature 6.988 (Cohen). The crystalline structure of the metal is well seen when tinplate is etched with hydrochloric acid containing free chlorine or with stannic chloride, a pattern (moire metallique) resembling frost pictures being produced. Crystallised tin is, however, best seen in the tin-tree or arbor Jovis, which is formed when a rod of zinc is suspended in a solution of stannous chloride (Ilsemann, 1786). Fine crystals of the metal are likewise obtained when water with zinc-dust in suspension is added to stannous chloride, as well as when this solution is electrolysed. The " cry " of tin, a creaking noise produced when a bar of the metal is bent, is due to the grinding of the crystals against one another. According to Gaubert macles - i.e. twinned crystals - of tin may be produced on the under surface of a smooth sheet of the metal when it is suddenly pierced by a needle, and the " cry " of tin is probably due to the formation of these macles. The folding or twisting of a sheet of tin also gives rise to twinned bands. Tin is harder than lead, but softer than gold; it is malleable at ordinary temperature, and can be beaten or rolled into sheet or foil; it is also ductile, increasing in ductility up to 100° C., but is not tenacious, a wire 0.08 in. in diameter breaking under a load of 54 lb.
At 200° C. tin becomes brittle and can be powdered. The mean coefficient of linear expansion of the metal between 0° C. and 100° C. is 0.00002296; its specific heat is 0.0524 at 18° C. and 0.0564 at 100° C., whence the atomic heat is 6.2 and 6.7 respectively. The thermal conductivity at 18° C. of block tin is 0.1453 gram calories, and of tin- wire 0.1549 gram calories; whilst the electric conductivities of these two forms of the metal at 18° C. are 8.28×104 and 8.82×104 units respectively. Tin obeys the law of Wiedemann and Franz that λ/K = constant, where λ is thermal and K electric conductivity, and also the law of Lorenz that this ratio is proportional to the absolute temperature.
As with many other metals, the melting-point of tin has been variously given. The most reliable results are those of Waidner and Burgess, who give the value 231.9° C. Other earlier but less reliable results are those of Heycock and Neville and Bogaderow. The former observers found the melting-point to be 231.5° C., the latter 231.14° C. and 231.25° C., whilst for the purest English tin, containing only a trace of iron, arsenic and phosphorus, Bogaderow found 230.92° C. Tin is said to volatilise between 1450° C. and 1600° C. (Carnelley and Williams), but its boiling-point, which is very high, is not known with accuracy. Greenwood gives the value 2270° C. Various alloys of tin have been fractionally distilled in the electric furnace. The spectrum of tin has been studied by Lohuizen, who has expressed the relationships between its lines by means of a formula.
The most intense lines in the spectrum of tin are as follow:
Arc: 2840.17, 2863.53, 3009.33, 3034.25, 3175.16, 3262.50, 3330.80, 3801.19, 4524.90.
Spark: 2840.10, 2863.33, 3175.15, 3262.48, 3801.32, 4524.90, 4585.80, 6453.00.