Alcohol compared to Water Revisited
Date: Fall 2009
In regards to your response in Alcohol vs Water
The original question was "When
rubbing alcohol evaporates, it 'feels' colder than water. Is this
because it requires more energy for a phase change than water?" Your
response was that rubbing alcohol requires more energy for changing
from liquid to gas, due to both the phase change (heat of
vaporization) as well as temperature change (specific heat capacity)
while in the liquid phase. Where does the different volatilities of
the two compounds come into this picture? I thought the increased
volatility of rubbing alcohol (and higher tendency for evaporative
cooling) was the main reason why it feels colder to the skin than water
This is the answer to which you are referring:
"You are correct in thinking that the difference in "feel" is a function
of the heat of vaporization. Iso-propyl alcohol (the main component of
rubbing alcohol) has a dH = 44 kJ/mol versus the dH of water which is
41 kJ/mol. Vaporizing alcohol requires more energy. However, another factor
is the specific heat of iso-propyl alcohol is 154 J/mol.K, that of water is
76 J/mol.K. This means for every degree temperature change in alcohol, a lot
more energy is absorbed."
When you say rubbing alcohol feels cooler than water, you are reporting the
experience that more heat is removed from the body by rubbing alcohol than
Volatility is the capacity of a substance to change from a liquid to a gas.
This characteristic is expressed by the heat of vaporization factor. So
your conclusion is accurate. This explains why the evaporation of rubbing
alcohol feels cooler than the evaporation of water.
I have read the original answer in the archives, and I won't argue
with its facts and figures. However, I think it may be misleading.
It quotes the molar enthalpies of vaporization of water and isopropyl
alcohol, notes that the value is higher for isopropyl alcohol, and
lets it go at that. What this means in English is that a molecule of
isopropyl alcohol absorbs more energy (heat) when it evaporates than a
molecule of water does.
However, the molecules aren't the same size. The molar mass of water
is 18 g/mol, while isopropyl alcohol is 60 g/mol, more than triple
water's. And note that their enthalpies of vaporization aren't very
different--that means that when 1 gram of water evaporates, it absorbs
nearly three times as much heat as 1 gram of isopropyl alcohol
Now, isopropyl alcohol boils at a lower temperature than water, and it
is more volatile than water at lower temperatures as well. This means
that its equilibrium vapor pressure is higher at all temperatures.
The effect of this is that if you have equal amounts of isopropyl
alcohol and water on your skin, much more of the alcohol will
evaporate over a short period of time. This means the evaporation
process will consume more energy per second. Although evaporating
equal masses of alcohol and water will require much more energy in the
case of the water, it consumes energy faster in the case of the
Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
Unfortunately "volatility" is a confusing term because it is used
colloquially to mean two different things. It is often used to
distinguish whether a substance has a lower boiling point, and it is
also used to mean that a substance requires less energy in order to
volatilize. Moreover, volatility is also a function of how strong the
intermolecular attractive forces are in the liquid. As such, we should
talk about boiling point versus evaporation, and the energy
requirements for the process.
While it is true that alcohol has a lower boiling point than water
(and will volatilize sooner given the same conditions), it is also
true that alcohol requires more energy become vapor (as mentioned,
because of its higher specific heat and enthalpy of vaporization).
Since we are essentially talking about a non-equilibrium condition of
evaporating a liquid (rather than having it boil), then the issue is
not whether alcohol reaches its boiling point sooner, but rather how
much energy is required to convert a liquid to gas. In this case,
alcohol requires more energy and therefore extracts more heat from our
skin and so feels cooler.
Greg (Roberto Gregorius)
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Update: June 2012