|MadSci Network: General Biology|
All liquids act as solvents. Water, for example, is an exceptionally good solvent. It dissolves ionic solids such as salt, as well as a few non- ionic solids such as sugar pretty well. It will not dissolve most non- ionic solids (like wax). Liquid hydrocarbons, such as octane, dissolve non-ionic solids like sugar, but don't dissolve ionic solids such as salt very well. When you think about the chemistry of solvents, it helps sometimes to think of the solvents and solids as members of different gangs. Ionic solids, like sodium chloride, wear red; they dissolve and mingle with other solvent molecules that also wear red. Non-ionic solvents wear blue; they only dissolve molecules that also wear blue. Mercury is unique; it is the only metal that is liquid at room temperature. Some metals, such as gallium, melt slightly above room temperature, while most metals melt at hundreds or even thousands of degrees above room temperature. When a metal is liquid, it can act as a solvent. Mercury, being a metal, is a pretty good solvent for metals, but not a good solvent for ionic or non-ionic molecules such as salt and sugar. If we think of the gang analogy, mercury wears green as itís gang color. All other metals are also green; they may dissolver more or less in the green-wearing mercury. When mercury dissolves ametal, the combination is typically called an amalgam (amalgam has also gained some uses not directly related to chemistry, but those meanings come from the chemistry and metallurgy of mercury compounds). Just like any solids when exposed to a solvent, different metals will dissolve to a greater or lesser extent in liquid mercury. Tin, copper, gold, and silver dissolve pretty well. Type 316 stainless steel (an alloy of iron, nickel, and chromium) doesn't dissolve very much; you can actually contain liquid mercury within pipes made from type 316 stainless steel. Once metals dissolve into liquid mercury, all sorts of chemical reactions between the metals can take place. Metals such as tin, silver, and copper will form compounds with the mercury. Those compounds generally have higher melting points than the mercury and lower solubility in the liquid mercury so, they precipitate out of solution. This is exactly what happens when a dentist uses a mercury amalgam to fill holes in your teeth. He blends liquid mercury with powdered metals, such as silver. The silver (and other metals) form compounds with the mercury so after a few minutes the mixture turns hard. The reaction is similar to what takes place with concrete; liquid water is mixed with calcium compounds; through chemical reactions, new calcium compounds precipitate out of solution, binding the sand and gravel together. The silver amalgam that the dentist puts in your teeth takes 48 hours or so to harden up. Therefore your dentist will tell you not to chew hard on that tooth for a day or so. If you donít listen, you can crack the amalgam pretty easily. There has been quite a bit of discussion about whether mercury in fillings poses a health hazard because of possible mercury vapor exposure. Most credible research seems to indicate that the mercury amalgams, when mixed and applied properly, do not pose a health hazard; there are those who strongly oppose this view and recommend that amalgam fillings be removed. Of course, while they are removed, the patient is exposed to more mercury fumes. I am a mad scientist; I donít have an opinion. Mercury has also been used for mining gold. The mercury is blended with sand and gravel containing microscopic flakes of gold. The gold dissolves in the mercury, while leaving behind the sand and gravel. When heated to high temperature, the mercury boils off, leaving behind the gold. Of course if you breath the mercury vapor, it will cause health problems (turn you too into a mad scientist), and releasing it to the environment is just wrong (let alone against the law). So, I hope that helps to answer your question about mercury amalgams.
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