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Name: Ronald B.
Status: other
Grade: other
Location: CA
Country: USA
Date: Summer 2009


Question:
Some years back, and acquaintance used to show his friends a trick using a piece of aluminum foil, such as from a gum wrapper, folded to about 1 square centimeter in size. He would dip it in a glass of tap water, place it in his palm and rub it a little, them offer anyone watching to place it in their palm. After a few seconds, the recipient would have to tip the aluminum off his palm because it was getting too hot, and if kept there would result in a serious burn. The little square of aluminum would continue to heat up and eventually would be only a small pile of material resembling ashes. The trickster would never reveal how this was done. Any ideas?



Replies:
The "trick" is that your acquaintance did not dip the aluminum gum wrapper into water, but rather, into an alkaline solution. Aluminum is an amphoteric metal, that is, it reacts both with acid and base. The heat of reaction of aluminum with base (hydroxyl ion) is about 9000 cal per gram of aluminum metal. That is a hugh amount of heat given that the heat capacity of a gum wrapper is very small. So the magician passes the gum wrapper to the unsuspecting volunteer when it begins to be uncomfortable. The volunteer gets to feel the full force of the increase in temperature and cannot hold the wrapper very long. What is left is a mixture of the paper and aluminum hydroxide. I predict that some foaming was also observed -- hydrogen gas. The reaction is: Al + (OH)^-1 + 3 H2O = [Al(OH^-1)4]^-1 + 3/2 H2(gas) A "chemical trick" unmasked!

Vince Calder


Hi Ronald,

Both water and oxygen can react with aluminum to produce large quantities of heat. The reason gum wrappers do not do so normally is that a small layer of oxide already exists on top of the pristine aluminum and this oxide layer prevents further contact with oxygen and/or water. By rubbing the foil vigorously, the layer of oxide is rubbed off and metallic aluminum (which was underneath) can now come into contact with either water or oxygen. The reaction of aluminum with oxygen is sped up at higher temperatures, so when the reaction begins and starts producing heat, it actually speeds up - further increasing the temperature and heat released.

Greg (Roberto Gregorius)
Canisius College


Hi Ronald,

My initial thought on your question was that something else must be involved. Aluminum foil dipped in water, regardless of folding or rubbing, should have this reaction. Neither simple foil nor those alloyed with or bonded to different commercial consumer products should do this. So something else must be involved. My guess is that it is something that reacts with the water or the foil directly.

I looked a little further and found what is probably the substance used. It seems that for some time people have been doing this trick using mercury bichloride, HgCl2. The substance reacts with the aluminum foil and produces the heat. The water is probably just a ruse, or at best is something that can be used to adjust the time over which the reaction proceeds. So your trickster would be putting a little of the mercuric chloride in the aluminum foil without your knowledge. The chemical used to be sold by magic shops, but due to its toxic nature is no longer available (I believe even illegal in the US). The mercury chloride can be adsorbed into the body with the potential for chemical burns and other serious health problems. (So do NOT try this!)

cheers,

Michael Pierce



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