This
is a neat 'magic trick' that illustrates the process of combustion, the
flammability of alcohol, and the special qualities of the material used
to make currency.
A combustion reaction occurs between alcohol and oxygen, producing heat and light (energy) and carbon dioxide and water.
C2H5OH + 4 O2 -> 2 CO2 + 3 H2O + energy
When the bill is
soaked an alcohol-water solution, the alcohol has a high vapor pressure
and is mainly on the outside of the material (a bill is more like fabric
than paper, which is nice, if you've ever accidentally washed one).
When the bill is lit, the alcohol is what actually burns. The
temperature at which the alcohol burns is not high enough to evaporate
the water, which has a high specific heat, so the bill remains wet and
isn't able to catch fire on its own. After the alcohol has burned, the
flame goes out, leaving a slightly damp dollar bill.
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