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How can evaporation occur?

by sgstudent
Tags: evaporation, occur
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sgstudent
#19
Feb5-13, 01:35 AM
P: 645
Quote Quote by AbsoluteZer0 View Post
So long as their kinetic energies average to get the specified temperature, yes.



Exactly.
Hi thanks for the help

What about melting could it be possible for those lucky bumps to occur as well allowing it to turn into a liquid?
nasu
#20
Feb5-13, 03:33 PM
P: 1,969
Quote Quote by sgstudent View Post
Hi thanks for the help

What about melting could it be possible for those lucky bumps to occur as well allowing it to turn into a liquid?
A solid can evaporate as well, at temperatures well below the melting point.
See "sublimation".
sgstudent
#21
Feb5-13, 10:59 PM
P: 645
Quote Quote by nasu View Post
A solid can evaporate as well, at temperatures well below the melting point.
See "sublimation".
But what about turning into a liquid before its melting point? Like how the liquid turns into a gas before it reaches its boiling point.

Thanks for the help :)
Lsos
#22
Feb6-13, 06:58 AM
P: 774
I'm no expert in this subject, but wanted to put my 2 cents into it. When a solid or liquid turns into a gas, the molecules escape (along with their energy) and are lost forever. I'm not sure about them turning into liquid (can one molecule even ever be considered a liquid?) but even if it does happen, it will, by definition, not "evaporate" away. It will instead remain on the surface, where it will turn back into solid almsot immediately.
jbriggs444
#23
Feb6-13, 07:43 AM
P: 907
Quote Quote by sgstudent View Post
But what about turning into a liquid before its melting point? Like how the liquid turns into a gas before it reaches its boiling point.
Consider the surface of water in a covered pot simmering on the stove. At boiiling temperature the rate of evaporation of water into the saturated vapor is equal to the rate of condensation of the saturated vapor back into the water. There is an equilibrium.

In the case of your clothing drying on the line in sub-zero temperatures the partial pressure of water vapor in the air is very low. So low that the boiling temperature of water at that pressure would be even further below zero. There is no equililibrium. The clothes dry out.

The point here is that the "boiling point" of a liquid is a function of pressure. In some sense, a liquid does not turn into a gas before it reaches its boiling point.

In the case of the transition between water and ice there is some pressure dependency. It is possible to melt ice by applying pressure rather than by increasing temperature. It is also possible to melt ice by applying salt. Also, some solids soften as their melting temperature is approached.
sgstudent
#24
Feb6-13, 08:09 AM
P: 645
Quote Quote by Lsos View Post
I'm no expert in this subject, but wanted to put my 2 cents into it. When a solid or liquid turns into a gas, the molecules escape (along with their energy) and are lost forever. I'm not sure about them turning into liquid (can one molecule even ever be considered a liquid?) but even if it does happen, it will, by definition, not "evaporate" away. It will instead remain on the surface, where it will turn back into solid almsot immediately.
Oh would the reason it turns into a solid immediately be that since it is at a liquid state where it does not have enough energy to escape like a gas. So it's own kinetic energy would be transferred to the other particles causing it to solidify again?
sgstudent
#25
Feb6-13, 08:24 AM
P: 645
Quote Quote by AbsoluteZer0 View Post
Yes.

[itex] E_k = \frac{1}{2} mv^2 [/itex]

What this equation states is that Kinetic energy is equivalent to half of the mass times the square of the velocity. When molecules collide with each other, they can either increase in velocity or decrease, changing their kinetic energy.



Molecules are always in motion and are always colliding with each other. There will always be a difference in kinetic energy between most molecules in a substance. One molecule in a substance may have the same temperature as another, but this commonality would be very short lived.
Hi AbsoluteZer0, I was thinking about the evaporation and when those few molecules gain enough kinetic energy and escape as a gas, what would their temperatures be? When they were still in a liquid state despite having more KE, the temperature still remained the same. Once they leave the liquid to becomes a gas, how will their temperatures considered to be?

Thanks for the help :)
Lsos
#26
Feb6-13, 08:36 AM
P: 774
Quote Quote by sgstudent View Post
Oh would the reason it turns into a solid immediately be that since it is at a liquid state where it does not have enough energy to escape like a gas. So it's own kinetic energy would be transferred to the other particles causing it to solidify again?
That's what I'm thinking. I didn't read this anywhere though, just arriving to conclusons...
AbsoluteZer0
#27
Feb6-13, 10:55 AM
P: 126
When they were still in a liquid state despite having more KE, the temperature still remained the same
Are you talking about the temperature of the whole system or the temperature of the individual molecules?
sgstudent
#28
Feb6-13, 11:00 AM
P: 645
Quote Quote by AbsoluteZer0 View Post
Are you talking about the temperature of the whole system or the temperature of the individual molecules?
I guess it would be those individual atoms? Since they have a greater kinetic energy so will they be considered to be at 100 degrees?
AbsoluteZer0
#29
Feb6-13, 11:10 AM
P: 126
Quote Quote by sgstudent View Post
I guess it would be those individual atoms? Since they have a greater kinetic energy so will they be considered to be at 100 degrees?
They wouldn't necessarily be at 100C. Evaporation can take place at temperatures lower than the boiling point.

http://en.wikipedia.org/wiki/Tempera...to_temperature

Classical thermodynamics concerns mostly the macroscopic (systems as a whole.)
As a consequence, it's inefficient to discuss molecules in terms of their thermal temperature. It's easier to discuss their statistical temperature. Statistical Thermodynamics deals with large populations of particles.

http://en.wikipedia.org/wiki/Statistical_mechanics


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