COMPOSITION OF ALLOYS OF COPPER IN PERCENTAGES
| Aluminium bronze | copper (90 to 97%), aluminium (3 to 10%). |
| Brass | copper (63 to 73%), zinc (27 to 37%). |
| Bronze | copper (70 to 95%), zinc (1 to 25%), tin (1 to 18%). |
| German silver | copper (56 to 60%), zinc (20%), nickel (20 to 25%). |
| Gold coin | copper (10%), gold (90%). |
| Gun metal | copper (90%), tin (10%). |
| Nickel coin | copper (75%), nickel (25%) |
| Silver coin | copper (10%), silver (90%). |
Electrotyping. Matter is often printed from electrotype plates which are prepared as follows. The matter is set up in type and wax is firmly pressed down upon the face of it until a clear impression is obtained. The impressed side of the wax is coated with graphite and the impression is made the cathode in an electrolytic cell containing a copper salt in solution. When connected with a current the copper is deposited as a thin sheet upon the letters in wax, and when detached is a perfect copy of the type, the under part of the letters being hollow. The sheet is strengthened by pouring on the under surface a suitable amount of molten metal (commercial lead is used). The sheet so strengthened is then used in printing.
Two series of copper compounds. Copper, like iron, forms two series of compounds: in the cuprous compounds it is univalent; in the cupric it is divalent. The cupric salts are much the more common of the two, since the cuprous salts pass readily into cupric by oxidation.
Cuprous compounds. The most important cuprous compound is the oxide (Cu2O), which occurs in nature as ruby copper or cuprite. It is a bright red substance and can easily be prepared by heating copper to a high temperature in a limited supply of air. It is used for imparting a ruby color to glass.
By treating cuprous oxide with different acids a number of cuprous salts can be made. Many of these are insoluble in water, the chloride (CuCl) being the best known. When suspended in dilute hydrochloric acid it is changed into cupric chloride, the oxygen taking part in the reaction being absorbed from the air:
2CuCl + 2HCl + O = 2CuCl2 + H2O.
Cupric compounds. Cupric salts are easily made by dissolving cupric oxide in acids, or, when insoluble, by precipitation. Most of them are blue or green in color, and the soluble ones crystallize well. Since they are so much more familiar than the cuprous salts, they are frequently called merely copper salts.
Cupric oxide (CuO). This is a black insoluble substance obtained by heating copper in excess of air, or by igniting the hydroxide or nitrate. It is used as an oxidizing agent.
Cupric hydroxide (Cu(OH)2). The hydroxide prepared by treating a solution of a copper salt with sodium hydroxide is a light blue insoluble substance which easily loses water and changes into the oxide. Heat applied to the liquid containing the hydroxide suspended in it serves to bring about the reaction represented by the equation
Cu(OH)2 = CuO + H2O.
Cupric sulphate (blue vitriol) (CuSO4ยท5H2O). This substance, called blue vitriol or bluestone, is obtained as a by-product in a number of processes and is produced in very large quantities. It forms large blue crystals, which lose water when heated and crumble to a white powder. The salt finds many uses, especially in electrotyping and in making electrical batteries.
Cupric sulphide (CuS). The insoluble black sulphide (CuS) is easily prepared by the action of hydrosulphuric acid upon a solution of a copper salt:
CuSO4 + H2S = CuS + H2SO4.
It is insoluble in water and dilute acids.