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Name: Alice
Status: other
Grade: 6-8
Location: LA
Country: N/A
Date: October 2006

Question:
Why does evaporation cause heat loss?



Replies:
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


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)


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


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

Peter Faletra


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


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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