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Rubbing alcohol, or isopropyl alcohol, typically is 70% alcohol and 30% water, and it also is available as 91% alcohol/9% water, which is why it doesn't burn. Denatured alcohol has no water in it, which is why it does burn. I use 91% rubbing alcohol as a wetting agent, in a spray bottle, for use in scenery-making because it evaporates fairly quickly vs. the water/detergent wetting method (which sometimes takes days). For track cleaning fluid on an RL LInes foam roller track cleaning car I use denatured alcohol. I've not used the 91% for stripping paint.  I also use 91% alcohol to lower the freezing temperature of windshield washer fluid.    Phil

Last edited by mtnhi7
Originally Posted by dwp425:

True, denatured alcohol does not have water in it ... but denatured alcohol is significantly different.  It it ethyl alcohol (2 carbon atoms) rather than isopropyl alcohol (3 carbon atoms).  Further, it is "denatured" with a few percent of other alcohols or solvents to render it undrinkable.

Ethanol and denatured ethanol both contain water, Ethanol when distilled forms a positive azeotrope and contains at least 4.73% water by weight.

Isopropyl alcohol also forms an azeotope with water.

Rubbing alcohol means that it is alcohol that is meant to be applied topically (on the skin).   It has nothing to do with the chemical composition of the compound.  You can find/buy "rubbing alcohol" that is ethanol based.  Most rubbing alcohols are based on isopropyl alcohol probably because it's cheaper.  


Denatured means that the alcohol is unfit for ingestion.  Some alcohols like methanol (aka wood alcohol) are "naturally" denatured because of their toxicity.  Isopropyl isn't nearly as toxic and has additives to make it less appealing.  Ethanol is what is in liqueur and it has the most additives to a) render it less appealing and b) to get an exemption from the drinking tax ;-)


The ratio of alcohol to water in % or proof (%/2) affects the flammability.  91% is extremely flammable and even the 70% stuff has a low flash point, 70 degrees.  91% only drops that down to 63 degrees.

Originally Posted by Tom Blevins:

Wow, after reading all the responses it's as clear as mud to me! Where is Bill Nye the Science Guy when you need him.

As a kid I remember watching Mr Wizard.

One of his experiments was to mix alcohol and water and then soak a handkerchief in it. The handkerchief was then lit on fire.

Only the alcohol would burn leaving the handkerchief untouched.

No TV magic there. I actually tried it a home and it worked.

(and no Timmys were harmed in the experiment )

Great thread that shows the amazing extent of knowledge on this forum.  I knew there were electrical and mechanical gurus, now I find out there are also a bunch of chemical ones too!   


Yea, looks like there should be either one less H molecule or it should be H8.  I did not even notice that until Chuck brought it up.  


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