Evaporation and Cooling
Date: October 2006
Why does evaporation cause heat loss?
I think the easiest way to think about this is that gas molecules have more
energy than liquid molecules because they move much faster (so they have
more kinetic energy). So for a molecule to enter the gas phase, energy must
be taken from the surrounding system and put into the molecule so it can
have enough energy to be in the gas phase. That is why when we sweat our
skin cools down, the energy that is needed for the sweat molecules to enter
the gas phase as it evaporates comes from our skin.
Stanford Department of Chemistry
in order for any molecules in the liquid phase to evaporate, that
molecule must acquire enough energy to break the intermolecular
attractive forces holding that molecule to the other molecules in
the liquid phase. This energy must come from the immediate
environment of the liquid and is usually going to come in the form
of heat transfer from the environment. Thus if the surroundings of
the liquid happen to have a higher temperature than the liquid, then
heat can be transferred to the liquid, and when the molecules
escape, the environment will have lost some heat.
Greg (Roberto Gregorius)
Atoms in a gas have more energy than molecules in a liquid. That
energy has to come from somewhere. As atoms evaporate, they absorb
energy from their surroundings. So, there's no overall heat loss; it
would be more accurate to think of it as heat transfer. The water
molecules evaporating from the surface of your skin absorb energy
from their surroundings (i.e., your skin) and consequently, the
atoms and molecules in your skin have less energy. In other words,
your skin is cooler.
Yes....evaporation is a cooling process. Water evaporating from your
skin helps cool your body...that is why we sweat.
In order for a liquid to become a gas, the liquid must get more
energy than it currently has. The best place for it to get this
energy is from its surroundings. This is seen as heat loss from the
object from which the liquid is evaporating. The total energy stays
the same, but now there is more energy in the gas than is in the
container that was holding the liquid. Think of boiling, it
requires a lot of heat to boil water. Think of boiling as very
quick evaporation--or conversely, think of evaporation as very slow boiling.
First, it is important that you understand the concept of a bell
curve. If I drew a graph for all the molecules in a glass of water,
I would want two factors on the graph. For the first axis,
(horizontal), we will use the temperature of any individual molecule
in the glass. For our vertical axis, we will use the number of
molecules at that particular temperature. The reason we call this
graph a bell curve is because it winds up shaped like a bell.
Most of the molecules will be right around the temperature we say
the water is at, but there will be a few that are much warmer or
cooler than the average.
When a molecule near the surface is warm enough, it will prefer to
be a vapor instead of a liquid. To simplify this a bit, think of
evaporation as the process of that molecule leaving. Well, since
only our warmest molecules will be leaving this way, the average
temperature begins to drop. Thus, we can call it a cooling process.
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Update: June 2012