Element Aluminium, Al, Poor Metal
|Of all the light metals, aluminium is the most widely distributed on the earth's surface. It forms a constituent of almost all crystalline silicate rocks; and of the secondary formations, the clays and slate formations are formed from aluminium silicate. A knowledge of the compounds of this metal, therefore, extends back as far as chemical knowledge at all can be traced. |
From the time of the discovery of the alkali metals, it was regarded as indubitable that a metal must be contained in clay. Wohler, however, was the first to obtain metallic aluminium by the action of sodium on the chloride. The method of separating the element from its compounds by electrolysis was given by Bunsen (1854).
The name aluminium is derived from alum (alumen), because aluminium is contained in this long-known salt.
At the present day, aluminium is prepared on a very large scale by the electrolysis of its oxide. The oxide is fused by the heat developed by the passage of the electric current, the aluminium goes to the cathode, and the oxygen which separates at the anode combines with the charcoal, of which the anode consists, to form carbon monoxide. To facilitate the fusion, the electrolytic vessel also contains other compounds of aluminium, e.g. cryolite (vide infra); since oxygen is more readily separated than fluorine (which is the corresponding other constituent of cryolite), this addition does not alter the chemical reaction, and only aluminium oxide requires to be thrown in to replace the used up material.
Metallic aluminium is a white, somewhat bluish metal which remains tolerably unchanged in the air. This is due to the fact that it quickly becomes covered with an invisible, thin, and firmly adhering layer of aluminium oxide, which protects the metal underneath like a varnish. It melts at 700°, and can be both cast and mechanically wrought into the shape desired, as it is not hard and is very ductile. Thus, thin wire and very thin foil, like gold-leaf and silver-leaf, can be made; the latter is greatly used for " silvering," since sulphurous gases do not blacken it. Aluminium is a good conductor for heat and electricity.
On account of its lightness (density = 2.7), its silver-like lustre, and its durability in the air, aluminium, especially since the electrolytic method has rendered it cheap, has become greatly used for ordinary utensils, but it does not seem hitherto to have been received with entire favour. This is perhaps to be accounted for by the fact that although it resists the action of pure water, it is rather strongly attacked by salt solutions of all kinds. Further, the oxidation of the metal generally occurs in spots, so that holes are there formed which can be repaired only with difficulty. Its resistance to mechanical action also is small.
When aluminium is alloyed with mercury, it appears to assume quite different properties. It is amalgamated by rubbing its surface with a mercury salt, e.g. mercuric chloride, with some pressure. The parts which were at first bright on account of the mercury, immediately become dull, and a moss-like growth of aluminium hydroxide arises from them. This phenomenon is explained by the fact that, although the protecting layer of oxide is formed at the amalgamated parts, the coating does not adhere, on account of the liquid nature of these, and the oxidation, therefore, pursues its course. It is not that the mercury produces an increased reactivity of the aluminium (a thing which is theoretically impossible), but the real chemical activity of the aluminium is allowed free scope to exert itself.
The amalgamated aluminium is employed as a reducing agent. On account of this behaviour, objects made of aluminium must be carefully protected from contact with mercury.
While, even at comparatively high temperatures, massive aluminium is only superficially and inappreciably attacked by oxygen, the finely divided metal burns with a brilliant light at a red heat. This can be shown by holding aluminium foil in the flame, or by blowing finely divided metal, such as is used in the form of aluminium bronze, through the flame. It takes fire, however, with greater difficulty than magnesium.
Aluminium dissolves in dilute hydrochloric and sulphuric acids with energetic evolution of hydrogen. In nitric acid it readily becomes passive, i.e. becomes coated with a layer which is not attacked by the acid, and then remains unchanged. Further, aluminium readily dissolves in a solution of caustic potash of soda, with evolution of hydrogen. This is due to the formation from the aluminium of an anion containing oxygen; we shall return to this later (vide infra). Salt solutions, also, especially solutions of ammonium salts, dissolve the metal fairly readily.
Aluminium forms alloys with various metals, and some of these are technically valuable. They will be mentioned under the respective metals. We would only mention here that an alloy (magnalium) has been prepared from aluminium and magnesium, which is stated to have technically valuable properties, and to be stable in the air.
History of Aluminium
|The term " alumen " was applied by the Romans to all bodies of an astringent taste, and among them alum was included. Alum was well known to Geber and the later alchemists, who erroneously classed it with the vitriols. This error was corrected by Paracelsus. The earth present in alum was for a long time supposed to be calcareous. In 1746, Pott stated that the basis of alum is an argillaceous earth, and in 1754, Marggraf showed that alumina and lime are two quite distinct earths, and that alumina is present in clay, combined with silica. |
By the early years of the nineteenth century alumina was regarded as the oxide of an unknown metal, which H. Davy unsuccessfully endeavoured to isolate. Aluminium was isolated by Wohler in 1827.