|MadSci Network: Chemistry|
There are many different metals with different properties. Some of them, like iron and lead and zinc, rust and tarnish quite readily in ordinary environmental conditions, so perhaps it is not even interesting asking about them. The four metals you have mentioned are ones that do not tarnish much: they certainly do not rust, or anything like that. But there are two different types of metal, and two reasons for not tarnishing, among the metals you have mentioned. Copper and silver are fairly unreactive metals, that do not react with ordinary acids, and do not readily react with oxygen. Both metals retain a fairly high polish, with no tarnishing, in many environments. But keep them out of the kitchen! Both copper and silver tarnish very readily in the presence of sulfur, or certain sulfur compounds. In the presence of rotten egg gas (hydrogen sulfide), or certain organic sulfur compounds, or even yellow elemental sulfur, copper and silver surfaces readily react to form black layers of copper or silver sulfide. These layers are very hard and insoluble; they are also rather unsightly! Cooking eggs is a notorious source of sulfur compounds that tarnish silver; so are natural gas or town gas fires. Both copper and silver also react readily with chlorine to form whitish powdery tarnishes, so swimming pool patios are perhaps another place to keep copper and silver away from! And copper (not silver) also reacts in the environment with carbon dioxide and rainwater, very slowly, to form a greenish grey coating of basic copper carbonate (verdigris). Greenish looking copperwork on building exteriors or statues is sometimes thought to be quite attractive, so much so that other chemicals are sometimes used to artificially hasten the process! Aluminium and magnesium are quite different. They are both actually very reactive metals. But these two metals (most especially aluminium) are protected from tarnishing in a special way. Aluminium reacts very rapidly with oxygen in air to form a surface coating of aluminium oxide. But the coating is quite different to the coating of iron oxide that forms when iron rusts. A rust coating is powdery and flaky, and when it forms, it exposes deeper and deeper layers of iron to be attacked by the rusting process. But the aluminium oxide coating is hard, invisible, and impervious. And it adheres tightly to the metal below. It just will not let other materials through to attack the underlying aluminium. It forms in a few seconds, and then the reaction stops. It only ever gets to be two or three molecules thick. If aluminium gets scratched, a new protective surface layer rapidly forms. In the absence of this surface layer, aluminium reacts rapidly and violently with air: machining aluminium is very dangerous, and has led to nasty fires; powdered aluminium is used in fireworks, and aluminium dust has caused very destructive explosions. The chemicals which tarnish aluminium, then, are unexpected ones. The metal is not easily attacked by acids or oxygen or chlorine because of the protective layer; The things that do make aluminium tarnish are things that dissolve or otherwise disrupt that surface layer. Alkalis are quite effective; powders that are used in automatic dishwashing machines are usually quite alkaline, and they will certainly help aluminium to tarnish by dissolving and disrupting that surface layer, and allowing the underlying metal to be attacked by oxygen. Aluminium cookware that is put into automatic dishwashers usually gets very tarnished. Mercury and mercury salts (** DANGER: very poisonous **) also disrupt the structure of the surface layer. Aluminium that has been scratched in the presence of liquid mercury, or dipped in mercuric chloride solution, rapidly grows a set of white whiskers of aluminium oxide when exposed to air again! Magnesium is also a very reactive metal that is protected by a surface layer in a similar way to aluminium. But the surface layer of magnesium dissolves in acids, and is unaffected by alkalis. John
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