Physical Properties of Copper

Copper is a metal of characteristic reddish colour, freshly fractured surfaces having a pinkish or yellowish tinge. When cuprous oxide is present, the colour of the fracture is purple-red. The metal crystallizes in octahedra belonging to the cubic system. Meunier observed that the introduction of a red-hot copper wire into the Bunsen gas-mixture does not ignite the gas, but causes the wire to glow, the copper becoming very brittle and apparently crystalline. The density of the pure electrolytic metal is D₄⁰=8.945, D₄¹⁵=8.9587, D¹⁰⁸³=8.40, D¹²⁰⁰=8.35; that of pure distilled copper after compression is D₄²⁰=8.9326 to 8.9376. Owing to its porous nature commercial copper has a lower density, 8.2 to 8.5. The melting-point is given as 1081° C., 1082.6° C., 1083° C., 1084° C., 1085° C., and 1103° C. The freezing-point is depressed by the addition of cuprous oxide, the eutectic being reached with 4.7 per cent, of oxide, and at 1065° C. The values stated for the boiling-point are 2310° C., 2180° C. at 257 mm., and 1980° C. at 100 mm. At white heat the molten metal emits a bluish-green light.

Copper is very tenacious, malleable, and ductile, and can be drawn into wire, or beaten out into thin leaves. When heated on glass, thin copper-leaf becomes transparent. According to Kahlbaum, the mean specific heat between 0° and 100° C. is 0-09272 (uncompressed) or 0-09266 (compressed). Magnus gives the value 0-0951 for the tem- perature-interval 15° to 238° C. As the result of twenty-seven determinations, Harper 16 found that the expression

0.0917+0.000048(t - 25) cal.

furnishes a basis for the calculation of the specific heat of copper at temperatures between 15° and 50° C. The electric conductivity of copper is almost as great as that of silver, the ratio at 13° C. being 100: 96.4. It has been suggested that copper can dissolve to a minute extent in water.

The possibility of the existence of a metastable form of copper has been discussed by Cohen and Inouye.

The molecular weight of the vapour has not been determined, but in solution in mercury, molten tin, and molten lead the molecule is monatomic.

Solid copper occludes hydrogen, but not nitrogen, carbon monoxide, or sulphur dioxide. Merton found that precipitated copper readily absorbs gases, which are expelled at high temperature. After a few weeks its power of absorption vanishes.

Molten copper absorbs hydrogen and sulphur dioxide, the occluded gases being eliminated on cooling. The liquid metal does not absorb nitrogen. It combines with oxygen to form cuprous oxide, so that fall of temperature is not attended by evolution of the gas. It decomposes hydrocarbons such as methane and ethane, with occlusion of hydrogen and separation of carbon.

According to Stahl, the absorption of gases by molten copper generally becomes greater with the temperature up to a certain point, with increase in the purity of the metal, and with the partial pressure of the gas. At 650° C. 100 grams of copper dissolve 0.1 milligram of hydrogen, and at 1500° C. 1.4 milligram, the solubility of the gas in both solid and liquid copper increasing as the square root of the pressure. The absorbed hydrogen has no influence on the conductivity of the metal. At 1420° C. 61 grams of copper absorb 0.15 c.c. of carbon monoxide, the physical properties of the metal undergoing a marked change.

At 800° C. and higher temperatures the ductility of copper is considerably increased by the presence of oxygen, but above 720° C. hydrogen has a weakening effect.

Electrolysis of a neutral or slightly alkaline solution of cupric acetate with a copper anode and a platinum cathode yields a deposit regarded by Schutzenberger as an allotropic form of copper. It is a very brittle, bronze-coloured substance of low specific gravity and high electric resistance. It readily undergoes atmospheric oxidation, and decomposes nitric acid with evolution of nitrous oxide. Schiitzenberger's original product contained cuprous oxide, and Wiedemann attributed its properties to the presence of this substance. It is possible, however, to prepare the product free from the oxide, although it always contains carbon and hydrogen. Benedicks regards it as a solid solution of acetic acid in copper.

An impure hydrosol of copper was first prepared by Lottermoser as an adsorption-compound with stannic oxide by heating a slightly alkaline solution of a cupric salt with a similar solution of stannous chloride in presence of an alkali-metal citrate or tartrate. With water the black precipitate yields a reddish-brown liquid, rapidly oxidized by atmospheric oxygen, with production of a greenish coloration which soon becomes yellow.

In dilute solution ammoniacal cupric sulphate is reduced by hydrazine hydrate, and cupric sulphate by hypophosphorous acid, the product of each reaction being an unstable hydrosol, which has a blue colour by transmitted light, and a reddish-brown colour by reflected light. The hydrosol is coagulated by prolonged heating, and is rapidly oxidized by exposure to air, with development of a green to yellowish- green coloration. Admixture with a solution of gum considerably increases the stability of the hydrosol.

Reduction of colloidal copper oxide in concentrated solution by hydrazine hydrate produces the unstable, blue liquid hydrosol, but in dilute solution in presence of ammonia the product is a stable, red liquid hydrosol. Evaporation of the red solution in presence of hydrazine hydrate yields a stable, solid red hydrosol. Other colloidal forms of copper combined with sodium lysalbate or protalbate have been prepared.

By Bredig's pulverization-method Billitzer has prepared a brown hydrosol, and Ehrenhaft a moderately stable olive-green to brownish-green hydrosol.