MadSci Network: General Biology

Re: why are amalgams solutions?

Date: Tue Apr 1 13:21:49 2003
Posted By: Joseph Weeks, President
Area of science: General Biology
ID: 1048211321.Gb

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 

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 

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.

Current Queue | Current Queue for General Biology | General Biology archives

Try the links in the MadSci Library for more information on General Biology.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2003. All rights reserved.