Aluminum, chemical symbol Al, is the most important element of group 3 of the periodic table. It is a light, silver-white metal 2.7 times as heavy as water, soft but with good tensile strength and an excellent conductor of heat and electricity. Aluminum melts at 1220°F (660°C) and is easily cast and pressed.
Aluminum’s other advantages are that it is also ductile-suitable for drawing into wire-and malleable, that is, easy to roll into sheets and foil. A structure fabricated from aluminum weighs approximately half as much as a similar steel structure of comparable strength.
Apart from its strength combined with its light weight, aluminum has another useful property-resistance to corrosion, because of a thin, hard oxide film that forms on its surface, protecting the metal from further oxidation. The oxide film can be thickened by anodizing, that is, oxidizing by an electrolysis process. The anodized film can be dyed, making aluminum ideal for architectural panels and household utensils.
Powdered aluminum is used in aluminum paint. In the powdered form, it is considerably more reactive than a solid block of metal and therefore is useful as a strong reducing agent for removing oxygen in chemical processes. When a mixture of aluminum powder and iron oxide is ignited, as in the thermite process, a large amount of heat is produced, and the iron oxide is reduced to molten iron. This technique is used in welding steel and iron and in incendiary bombs.
Occurrence of aluminum
Aluminum is the third most abundant element, after oxygen and silicon in Earth’s crust, making up about 8 percent of the total. Iron, the next most abundant element, is only five percent of the total. Like so many of the metals, aluminum is not found in its pure form but associated with other elements in igneous rocks, and minerals. An aluminosilicate such as feldspar, KAlSi3O8, is the main constituent of many rocks such as granite, which is quartz and mica cemented together with feldspar. These rocks are gradually weathered and broken down by the action of carbon dioxide from the air dissolved in rainwater, resulting in the formation of kaolin, china clay, Al2Si2O5(OH)4. Further weathering ultimately gives bauxite, Al2O3H2O or Al2O33H2O, which is a hydrated form of aluminum oxide occurring widely and used for commercial aluminum extraction and refining.
Pure aluminum oxide, also known as alumina, Al2O3, is found as corundum, a crystalline, extremely hard mineral. It also occurs combined with magnetite (iron oxide), a form known as emery. Both are used as abrasives. Traces of other metal oxides present in aluminum oxide tint it to form precious stones. Chromium gives a red coloration to ruby, whereas cobalt accounts for the blue color of sapphire.
Uses of aluminum compounds
Crystalline alumina is used as an abrasive and, in powdered form, for column chromatography, an analysis technique in which a liquid mixture of compounds is allowed to trickle down through a glass column packed with powdered alumina, causing the various compounds to separate at different levels. Aluminum hydroxide, Al(OH)3, is used as a mordant in dyeing. A fabric that will not accept a dye is impregnated with the mordant. The dye reacts chemically with the mordant to form an insoluble "lake," or pigment, thus dyeing the fabric. Aluminum hydroxide dissolves in acids to form salts and in alkalis to form aluminates-few substances do both. Sodium aluminate, NaAlO2, is used as a flocculating agent during water- and sewage-treatment processes, coagulating impurities. It is also used in papermaking.
Aluminum sulfate, Al2(SO4)318H2O, is widely used as a source of aluminum hydroxide. It is also used together with sodium carbonate in foam fire extinguishers. It forms double sulfates with other metals. These substances are known as alums, for instance potassium alum, K2SO4Al2(SO4)324H2O. Alums are used in manufacturing processes (such as dyeing), during the papermaking process, and in the production of medicines, textiles, and paints.
When chlorine gas is passed over aluminum foil, a white solid, aluminum chloride, Al2Cl6, is formed, an important catalyst in the synthesis of aromatic compounds.
