Physical Properties of Zinc
|Zinc is a greyish-white or bluish-white metal. It has a metallic lustre, can take a high polish, and tarnishes quickly in ordinary air. |
Zinc is said to undergo allotropic modification from an a-variety to a β-variety at 174° C., and from the β-variety to a γ-variety at 322° C. According to Cohen and Heldermann, ordinary zinc is a metastable mixture of several allotropic modifications, and " atomised " zinc is also a mixture of two or more allotropic forms.
Zinc sublimes in a crystalline form when it is distilled in hydrogen or in vacuo. The crystals were supposed to occur dimorphically in the regular and hexagonal systems, but crystalline zinc is, however, apparently hexagonal and probably holohedral.
When zinc is crushed its micro-structure becomes homogeneous and its hardness increases. Annealing restores the crystalline structure. The cast metal " cries " more feebly than tin.
It is brittle at ordinary temperatures, but becomes malleable and ductile at 100°-150° C. The metal is rolled hot on the large scale, but it remains sufficiently malleable after heating to be rolled or drawn into wire. Above 205° C. the metal again becomes brittle enough to be pulverised in a mortar.
Zinc is distinctly plastic at 200°-400° C., and the warm metal can be pressed through an opening.
The modulus of elasticity, or Young's modulus of zinc, has been given as varying between 7670 and 10,550, or an average of 9300 kgm. per sq. mm.: it varies with the nature and purity of the metal.
In determinations of the tensile strength the breaking load of thin rolled zinc was found to be about 24,000 lb. per sq. inch, and the tension modulus of elasticity 11,500,000 lb. per sq. inch.
The average compressibility of zinc, the fractional change of volume produced by one megabar pressure, is 1.5×10-6 per unit volume per megabar, between 100 and 500 megabars.
The hardness of zinc seems to be 2.5 on Mohs' scale, but it varies with the purity of the metal, and perfectly pure zinc seems to be softer than silver.
The density of distilled zinc at 20°/24° C. is 6.9225, which rises to 7.12722 after compression under 10,000 atmospheres, but it varies with the history of the metal and usually diminishes on "working."
Schiff found that the density of granulated zinc at 12° C. varied from 6.966 to 6.975, and recorded determinations by other observers from 6.861 to 7.1908. According to Kalischer, a specimen of rolled zinc of density 7.1812 had a density of 7.1841 when it became crystalline by heating to 130°-300° C. (Water at 0° C. = 1.)
The density of zinc decreases on melting. One grm. of zinc was found to expand by 0.010 c.c. on melting, the corresponding densities have been estimated at 7.2 and 6.48, and the density of molten zinc is expressed by the formula
D = 6.59 – 0.00097 (t – 419)
at any temperature t. The slight contraction during solidification adapts the metal for castings.
The vapour density of zinc corresponds to a monatomic molecule. When zinc dissolves in mercury it depresses the vapour pressure as if its molecule were monatomic.
Zinc melts at 419.4° C. There has been a gradual convergence on this figure during the progress of research.
The latent heat of fusion of zinc is about 1.730 Cal.
Berthelot found 920° C. for the boiling-point of zinc. Higher temperatures had been found by earlier investigators. More recently 918° C. has been assigned as the boiling-point.
In vacuo zinc volatilises slowly at 184° C. and boils at 550° C. According to Heycock and Lamplough, the boiling-point alters by 0.133° C. per mm. difference from normal pressure.
The latent heat of vaporisation at boiling-point is 31.430 Cal. The specific heat varies from 0.08421 at -127.5° C. to 0.09570 at 123.5° C. From the mean of determinations by Naccari, Bede, and Schliibel, the specific heat of zinc is 0.0929 between 18° C. and 100° C., 0.0957 between 18° C. and 200° C., and 0.0978 between 18° C. and 300° C. According to Kahlbaum, Roth, and Siedler, the specific heat of distilled zinc is 0.0939, which becomes 0.0940 after compression.
The thermal conductivity decreases with the temperature up to the melting-point. Then there is a sharp drop, succeeded by another gradual rise. According to Lees, the thermal conductivity of pure, redistilled cast zinc varies from 0.20 at -170° C. to 0.268 at 18° C. According to Jager and Diesselhorst it is 0.265 at 18° C. for pure cast zinc and 0.262 at 100° C.
The electrical conductivity in reciprocal ohms per cm. cube varies from 19.5×104 at -170° C. to 16.9×104 at 18° C. The conductivity has been expressed as 5.45 (l+0.0039t+0.0000017t2) microhms at any temperature t between 15° C. and 300° C.
Breaks in the thermal and electrical conductivity curves have been connected with allotropic modifications of zinc.
The coefficient of linear expansion is 10.06×10-6 between -183° C. and 12.6° C., and 17.11×10-6 between 19.3° C. and 100.2° C.
The most important lines in the arc spectrum of zinc, in Angstrom (10-8 cm.) units, are: 3036, 3072, 3345, 4630.06, 4680.138, 4722.164, 4810.535, 4912, 4925, 6103, 6362.345.
|A fairly stable colloidal solution of zinc in ether has been obtained by sparking between zinc electrodes immersed in the liquid. |
Colloidal solutions can be prepared by electrical discharge between zinc electrodes under water, but they are more unstable than corresponding cadmium solutions.
The colloidal solution of zinc in isobutyl alcohol, obtained by an electrical method, is stable. It is brownish red by transmitted light and greyish black by reflected.