When a burning agent, such as pepper spray or alcohol or bleach or whatever, touches the skin or eyes, is there any transfer of heat? Or are the nerves just tricked into thinking that they are being burnt?
Conti
Depends how you mean. Rubbing alcohol genuinely generates heat when mixed with water, so will generate actual heat when rubbed on the skin. Concentrated acids and some bases will also generate heat when they come into contact with your skin. Mostly though the burning sensation you get is simply the pain receptors firing because they have been chemically triggered, not by an actaul temperature rise.
The following article just discussed some of this:
The related threads are still active and are discussing the question in more depth:
http://boards.straightdope.com/sdmb/showthread.php?threadid=47700
http://boards.straightdope.com/sdmb/showthread.php?threadid=49334
I suggest you join in on one of the threads as I don’t want to have to repeat the information on different threads.
Hmmm.
I think that when the rubbing alcohol is applied on the skin it evaporates. The act of evaporation requires heat energy which is taken from the skin (? more properly called heat of vaporization). This cools down the skin.
Karlgauss,
Alcohol evaporates more rapidly than water, cooling the skin, but when rubbing alcohol is mixed with water it actually generates heat. That’s the reason behind it’s use for sporting injuries. Rubbing it on your skin activates the sweat glands and heat is generated. You can try this yourself. Put some water in your hand and pour in some rubbing alcohol and feel the temperature rise. I think aftershave would work just as well, it’s all isopopanol isn’t it?