Explanation:

This is a nice "trick" which illustrates combustion, vapor pressure, and the flammability of alcohol.

Lesson:

A dollar bill is soaked in a mixture of isopropyl alcohol and water, the bill soaks up the water and the alcohol remains on the surface. When the bill is lit, the alcohol burns very fast while the water protects the dollar bill.

Advanced Lesson:

The combustion reaction occurs between oxygen (O) and isopropyl alcohol (C₃H₈O) producing carbon dioxide and water. It is an exothermic reaction so it also produces energy in the form of heat and light (fire).

C₃H₈O + 5O₂ -> 3CO₂ + 4H₂O + energy

Even though the dollar bill is called "paper money," it's actually made out of fabric, a weave of cotton and linen (which is nice if you've ever accidentally left one in your pocket during a wash). When immersed in the water-alcohol mixture the fabric soaks up the water. Since alcohol has a higher vapor pressure than water, it evaporates more readily and mainly inhabits the surface of the fabric. When the bill is lit, it is the alcohol which actually burns. The temperature at which alcohol burns isn't hot enough to evaporate the water so the bill remains wet and never catches fire.

Closer Look

HowStuffWorks: How Money is Made
http://www.youtube.com/watch?v=O1bHQKdLGXg

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