|MadSci Network: Chemistry|
Hi Steve, This is an interesting question. To help myself understand what you were seeing I attempted to reproduce your results in the lab where I work. I tried three or four times each with two pennies and was unable to cause the glow that you observed. (I will get back to why I think my attempts didn't work). Here is what I think is happening: The hot penny is igniting the acetone fumes mixed with air above the beaker and causing them to burn at the penny surface. I suspect the reaction would work on almost any metal surface but is more observable against the dark background of the penny (as opposed to the light reflective background of a quarter which has a surface layer composed of 75% copper according to the CRC handbook of chemistry and physics 1989-90, pg B-35). I didn't find any examples of copper assisted combustion, but it is certainly well known that some metals are better for this kind of reaction than others (hence the expensive platinum and palladium in catalytic converters rather than cheap copper!). The balanced chemical equation for combustion of acetone is: 4 O2(g) + 1 C3H6O(g) ---> 3 H2O(g) + 3 CO2(g) + heat. As for my own experiments, my lab is quite drafty due to the ventilation system we have so I think the acetone/air mixture outside the beaker was too thin for ignition (demonstrating that the ventilation system is doing the job it is supposed to do) and the mixture inside the beaker was too rich. I am sure an open flame would be required under those conditions. I also found that a penny cannot be directly heated to a temperature at which it glows without it first melting into a glob of metal (which I infer from your question is not what happens in your case). This is further evidence that the glow you are seeing is the combustion reaction and not from the temperature of the metal. I hope this information was helpful! Jeremy.
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