Evaporation and Cooling

name         Alice
status       other
grade        6-8
location     LA

Question -   Why does evaporation cause heat loss?
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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.

Ethan Greenblatt
Stanford Department of Chemistry
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Alice,

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)
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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.

Paul Mahoney
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Yes....evaporation is a cooling process. Water evaporating from your 
skin helps cool your body...that is why we sweat.

Peter Faletra
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Alice,

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.

Matt Voss
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Alice,
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.

Ryan Belscamper
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