I am just wondering what the mechanism is that causes water to “dry” (=evaporate?)
As I understood it for science, water becomes steam at 100c. How exactly is this different to evaporation?
I know I should know this, but somehow I don’t.
I’ll try a simple answer, but it’s a while since I was doing physics at Uni, so I can’t guarantee correctness.
At 100c all the water molecules have sufficient energy to break their inter molecular bonding and move from liquid to gas stage. This is quite different to evaporation which of course doesn’t need the water to be at 100c. To understand evaporation you must consider the water at a surface with air above the surface. Both the water and the air will contain molecules of water, and these molecules will each have a random kinetic energy. Because the molecules are moving a lot, some will move from the water into the air, and some will move from the air into the water. Depending on the warmth of the water, and the amount of water molecules in the air the number of molecules moving from air->water will be different to the number moving from water->air.
OK so far?
So if the air is dry (has few water molecules in it) and the water is warm (the molecules in it have more kinetic energy) then more molecules will move from the water to the air than from the air to the water. This will lead to the total amount of water molecules in the liquid state getting smaller and smaller until the puddle dries up.
Cheers, Bippy
It’s actually a very interesting process.
Temperature corresponds to the average kinetic energy of the substance in question. Kinetic energy is directly proportional to the square of the velocity of the particles that make up the substance. The distribution of kinetic energies of the particles is described by what is called a Maxwell-Boltzman distribution diagram. With me so far?
Anyway, at any given temperature, even far below the boiling point, some particles will have a high enough kinetic energy (and therefore) velocity to break the intermolecular bonds between them and the surrounding water molecules. Of course, for a given water molecule to escape, and thus enter the gaseous state, it must also be near the surface of the water, or it is likely to collide with another water molecule, likely slow down, and thus remain part of the liquid.
As the temperature of the water increases, the distribution diagram “shifts to the right”, i.e. more water molecules have the requisite kinetic energy to escape, and the rate of evaporation increases.
Interestingly, if the liquid water is contained in a container that limits the transfer of heat, the temperature of the remaining liquid water will drop, since the molecules with the highest kinetic energy will be lost. On the other hand, if there is good heat transfer between the water and its surroundings, and assuming the water is initially at the same temp as the surroundings, heat will necessarily flow from the surroundings to the water so as to maintain a constant temperature. The evaporation of water is thus an endothermic process.
Next lesson: put a lid over the water, and we can talk about “vapor pressure.”
There: all from memory!
–robby (who wishes he was still teaching…)
P.S. don’t ever think water is simple, my physics professor and mentor would say that (paraphrasing)“If you ever feel we nearly understand all about physics, just remember many people are still researching the properties of water, and we are far from knowing them all yet.”
I remember fairly recently reading that those researchers discovered a metallic phase of water near absolute zero
Crap! Beaten by a minute! (See what I get for taking the time to spell-check?)
To follow up with Bippy’s first post, the rate of evaporation (liquid to gas) is dependent ONLY on the temperature of the water.
The rate of condensation (gas to liquid) is dependent ONLY on the partial pressure of the water vapor.
The rate of NET evaporation is equal to the former minus the latter.
If the container of water is covered, the partial pressure of the water vapor will increase until the rate of condensation equals the rate of evaporation. The partial pressure will then remain constant. We call this the vapor pressure of the water. It is dependent only on the temperature of the water. The only requirement is that some liquid water remain in the container. Vapor pressure is independent of the size and shape of the container, nor does it depend on the surface area of the liquid water.
Indeed. Researchers are still investigating the physics at the molecular level of how ice skating works, for example.
Never heard that regarding water (though there are multiple ice phases at various temps/pressures that have been studied going back decades).
I have heard of metallic hydrogen, however, at extreme pressures.
Very good post, but not entirely correct.
Rate of Condensation also depends on :
1> thermal conductivity of the air
2> whether the air above is still or circulating.
3> temperature of the air (if the condensing vapor freezes it makes frost which changes thermal conductivity)
4> Total pressure - because that plays an important part in deciding the diffusion rate of vapor to the surface.
There can be others too. But generally speaking u r quite on the mark.
Thanks! 
To address most of your concerns, I am assuming:
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The temperature of the vapor phase is identical to that of the liquid phase.
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The vapor phase is homogeneous.
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Ideal (perfect) gas assumptions hold for the vapor phase.
Under normal, room-temperature/pressure conditions, I believe most of these assumptions are valid enough so as not to present much of a problem.
For robby and others.
I checked up about a metallic water phase, since I’d only heard about it.
Here is a link to a paper Meatllization of Hydrogen and Other Small Molecules at 100 GPa Pressure
Cheers Bippy