How does evaporation work at the molecular level?

[andrewbb]

The moderators here decided my explanation of evaporation was wrong, but they failed to explain why. They simply said I was an idiot and locked my thread.

So... I ask the gods of physics here that moderate this board: How does evaporation work at the molecular level?

Say you have a body of water at the same temperature as the surrounding air. No heat added whatsoever.

My questions:
- does evaporation occur?
- if so, what causes that evaporation and what is happening at the molecular level?

 

[vanesch]

Be careful when re-posting a thread that was, for clear reasons, closed !

But OK, let's consider your question "what is evaporation" under the PoV that you are asking a question and not trying to find a back door to propose a "new theory".

As this is something pretty elementary, the wiki article on evaporation seems like a good starting point. http://en.wikipedia.org/wiki/Evaporation

From there:

For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces.[1] Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. This phenomenon is also called evaporative cooling.

looks to me like a good qualitative starting point. Is that sufficient as a start of an answer to your question ?

 

[cesiumfrog]

The moderators here decided my explanation of evaporation was wrong, but they failed to explain why. They simply said I was an idiot and locked my thread.

Yeah, welcome to PF.

Evaporation isn't some kind of interaction between water and air molecules. Apparently that's a common misunderstanding, but evaporation is basically independent of the air (and type thereof) and occurs fine without any air at all. You kept returning the discussion to details you invented for that interaction - some kind of "electric attraction" and a "balloon effect" (which doesn't even make sense.. the buoyancy of a balloon depends on excluding some density whilst such attraction would only raise it).

 

[andrewbb]

That explanation of evaporation is good, but doesn't describe the detail at the molecular level.

Water molecules are polar which means they are basically tiny magnets. The two hydrogen atoms sit on one side of the oxygen atom creating a positive charge on one side and negative on the other.

For a single molecule to evaporate it must overcome the cohesive force of the water (hydrogen bond). Hydrogen bonds are essentially magnetism. (The hydrogen is electromagnetically attracted to the oxygen in another molecule.) While heat excites the molecules and churns them more vigorously, evaporation can happen in two ways:
1. two water molecules align so their oxygens repel from each other.
2. a water molecule hydrogen bonds to a molecule or atom in the air.

Does anyone have an argument to that way of looking at it? We are talking about the same thing, I am merely describing what happens to the individual water molecules. The temperature is a measurement of how vigorously the molecules are moving. If the molecules are moving around more vigorously, their hydrogen bonds are more likely to be broken which allows them to escape into the air.

 

[DaveC426913]

1. two water molecules align so their oxygens repel from each other.
2. a water molecule hydrogen bonds to a molecule or atom in the air.

Does anyone have an argument to that way of looking at it?

Yes. Both are wrong.

Evaporation has nothing to do with the molecular structure of water. (Yes, there is a small factor of the water's polar structure, but that an influencing factor, not a determining factor. Lots of non-polar liquids are perfectly capable of evaporating).

So you can think of water molecules as merely atoms. They bump into each other, one gets more kinetic energy and another gets less. The one that gets more energy has enough to escape.

That's it.

 

[andrewbb]

You are saying the molecules are bumping into each other. I agree. I am describing exactly HOW they are bumping into each other.

If the H bumps into an O and they don't bump too hard, they will stick together electromagnetically (hydrogen bond).

If the O bumps into an O they will repel from each other. Partly Newtonian, but the negative charge of the O's will ADD to the repulsion.

Do you disagree with that?

 

[Char. Limit]

Well, no. The problem is that you're assuming that was the only factor. If that was true, non-polar molecules (such as gasoline, made up of various hydrocarbons of formula C8H18) would never evaporate. We know that gasoline evaporates, so your idea must either simply be a contributing factor (NOT the entirety) or false.

 

[andrewbb]

My point is that the orientation of the molecule is the key to understanding what is happening as the molecule overcomes the cohesive force of the water. If the water is moving vigorously (high heat), then more molecules escape that cohesion. They are bouncing at odd angles and the electrical charges of the molecules as they collide are an important aspect to evaporation. If you have 4 H's colliding, they will magnetically repel from each other. If you have 2 O's colliding they will magnetically repel from each other. If a single H and a single O collide, then the velocity of the collision will determine if they bounce off or stick electromagnetically (hydrogen bond).

 

[andrewbb]

Being non-polar does not mean the molecule doesn't have an electrical charge to it. Water just happens to have an electrical charge on each side.

 

[andrewbb]

At high heat, I do agree the energy level of the water molecule will cause it to spontaneously overcome the hydrogen bond (cohesion) to other water molecules.

However, at lower heat overcoming that bond is more difficult. That is where the electromagnetic repulsion/attraction of the water molecules becomes important.

So.. assuming low heat: much of the evaporation is caused by the repulsion effect of 2 O's or 4 H's pushing away from each other. In the case of gasoline, and assuming the temperature is the same in both bodies of fluids (air and gasoline), I posit there is an electromagnetic attraction to another molecule in the air that is responsible for the evaporation. If you disagree, and assuming the same temperature, where does the energy come from to evaporate the water/gasoline (separate it from its cohesive bond)?

 

[DaveC426913]

You are continuing to assume that the polarity plays a critical role in evaporation.

There is no point in you proceeding until you determine just how much the property of polarity affects evaporation.

Let's resolve that first.

 

[Char. Limit]

First, I assume that by heat, you mean temperature. In chemistry, heat is an entirely different concept.

Second, you will have to define "low temperature" and "high temperature". Remember that even ice water has temperature of 273.15 K. It doesn't matter what the temperature of the outside is: that only determines the change
in the change in temperature, or if you are
fluent in calculus, the temperature of the surrounding air only determines the second derivative of the water temperature.

Water at room temperature has close to 3,750 J. That's significant kinetic energy, no matter the surrounding temperature.

 

[andrewbb]

The electrical charge plays a critical role in evaporation only at low temperatures. At higher temperatures, the kinetic energy is certainly more important.

 

[DaveC426913]

Not a reliable source but you can do your own research:

At BP, the transition of phase, or evaporation, occurs. The rate of evaporation entirely depends on the rate of heat input or the rate the pressure drops, which is the removal of the gas from the liquid. This process has nothing to do with the substances molecular structure.

http://answers.yahoo.com/question/index?qid=20080928154829AApFBXa

 

[andrewbb]

I don't know what temperature the electrical charge becomes more important. Certainly at 212 F and the electrical charge effect is nearly entirely negated. At room temperature? It becomes a factor.

At room temperature, both bodies of fluids are the exact same temperature. The cohesion of water is quite strong. What is responsible for a single water molecule to evaporate? It's not moving fast enough to overcome the cohesion, so for it to have the energy to separate from the bond, it must be either repelled or attracted to something. EG. tumbling water molcules. some of which are hydrogen bonded to each other will align in certain ways to attract/repel from each other based on their charges. That's where I think the energy comes from to evaporate.

And I don't see any reason why the electrical charge of water wouldn't be attracted to the electrical charge of a molecule in the air. That's certainly a factor, given the same temperature and no introduction of another energy source.

 

[hamster143]

If you disagree, and assuming the same temperature, where does the energy come from to evaporate the water/gasoline (separate it from its cohesive bond)?

It's already there. An average speed of water molecules at room temperature is in hundreds of meters per second. Intermolecular bonds keep most molecules inside the main body of water, but some of them occasionally attain enough energy, simply through random interactions with each other, to break the bonds and fly away.

Air is irrelevant. You'll observe evaporation even in the vacuum.

 

[Char. Limit]

We're not saying that dipole-dipole attraction (what you're describing) isn't a factor. We're saying that at room temperature, it isn't significant.

Also, please, don't use Fahrenheit. Celsius is ok, but any real scientist in my mind should use Kelvins. It gives you the idea of how much energy room temperature really is. Room temperature is 295 Kelvins. That's a lot, considering that the boiling point of water is 373 Kelvins. 295/373 is highly significant fraction of energy.

 

[andrewbb]

Describing "phase" or "state change" of water simply because it is in liquid water or floating in the air seems erroneous to me.

The water molecule is identical in both places. The temperature can be identical. The only difference is its surrounding molecules. To say it's phase is different in that case is not very descriptive.

 

[Char. Limit]

If the temperature (and pressure) are identical, the vast majority of water molecules will be in the same phase. I'm talking 999 out of 1000.

 

[cesiumfrog]

andrewbb, on a molecular level evaporation is very simple: some water molecules on the surface happen (after random collisions with their neighbours) to have enough kinetic energy to break away from the attraction of other water molecules (think escape velocity).

Not only was your air-water attraction theory wrong, but if anything it was closer to the (opposite) process by which some of the air molecules (including H2O from the vapour phase) get dissolved into the water.

 

[andrewbb]

andrewbb, on a molecular level evaporation is very simple: some water molecules on the surface happen (after random collisions with their neighbours) to have enough kinetic energy to break away from the attraction of other water molecules (think escape velocity).

Not only was your air-water attraction theory wrong, but if anything it was closer to the (opposite) process by which some of the air molecules (including H2O from the vapour phase) get dissolved into the water.

I agree, some molecules will be dissolved into the water, but to say molecules aren't attracted to each other's electromagnetic charges is not reasonable.
- H20 has two electrical charges and is essentially an oddly shaped magnet.
- Other atoms/molecules also have electrical charges (and odd shapes).
- To say that H20 won't temporarily be attracted to another atom/molecule due to their electrical charges is illogical.
- H20 is light enough to float in air. If attached to another atom/molecule, it WILL have a balloon effect.



Also.. the cause of those "random" collisions of water molecules is not random. It is as I described. Assuming no other external current or heat, the movement of water molecules is a result of the electromagnetic charges of the water molecules. Water molecules are essentially magnets. If you disagree, what other cause for water's movement is there? No heat. No current. Saying "kinetic energy" is not descriptive. What is happening?

Imagine a flat enclosure in the shape of a circle. Fill it with an odd number of north-south pole magnets. Those magnets will be sliding around for quite some time (until they find equilibrium). Especially if you have oddly SHAPED magnets such as H20. And it's possible you'll get a few of those magnets jumping out (evaporation).

 

[Dale]

If the H bumps into an O and they don't bump too hard, they will stick together electromagnetically (hydrogen bond).

If the O bumps into an O they will repel from each other.

Actually, the orientation is pretty much irrelvant. If two otherwise isolated water molecules collide with low enough kinetic energy to form a hydrogen bond then they will simply rotate into the minimum energy orientation regardless of their initial orientation. They are not constrained to always maintain a certain orientation.

Also, the dipole field of a water atom is electrostatic, not magnetic. If it were magnetic then water would jump onto a magnet.

 

[andrewbb]

Actually, the orientation is pretty much irrelvant. If two otherwise isolated water molecules collide with low enough kinetic energy to form a hydrogen bond then they will simply rotate into the minimum energy orientation regardless of their initial orientation. They are not constrained to always maintain a certain orientation.

I agree. If they are moving slow enough, the electromagnetic attraction will re-orient the molecules and form a hydrogen bond. But of course, if the two O sides hit each other the molecules will be more likely to repel. Of course, they could turn and form a hydrogen bond, but it entirely depends on the angle and velocity at which they collide.

 

[Dale]

Of course, they could turn and form a hydrogen bond, but it entirely depends on the angle and velocity at which they collide.

No, it depends only on the velocity, not the angle.

 

[andrewbb]

No, it depends only on the velocity, not the angle.

I disagree.

You've got a molecule that's basically shaped like mickey mouse. Two hydrogens on one side, an oxygen on the other.

Imagine sliding two spinning mickey mouse magnets against each other on a table. You're saying the angle at which they collide doesn't matter as to where they end up? Those magnets are going to bounce all over the place and only possibly will the O and H line up and stick.

 

[Dale]

Assuming no other external current or heat, the movement of water molecules is a result of the electromagnetic charges of the water molecules. Water molecules are essentially magnets. If you disagree, what other cause for water's movement is there? No heat. No current. Saying "kinetic energy" is not descriptive. What is happening?

Do you understand that for any material which is not at absolute 0 temperature the atoms are constantly moving? This is called thermal motion.

The higher the temperature of the material the faster the average velocity (higher KE) of the atoms, but there is always some statistical spread of velocities. The energy of individual atoms follows the Boltzmann distribution.

 

[Dale]

You're saying the angle at which they collide doesn't matter as to where they end up?

Yes, more or less. I am saying that the angle at which they collide doesn't matter as to whether or not they stick together, only the speed matters.

 

[DaveC426913]

Andrewbb: when water molecules stick together we call that ice. In liquid water, they do not stick together; they act more like regular atoms.

The bipolarity is just not that big of a factor, no matter how much you wish it so.

Stop hypothesizing.

The behaviour of water at room temperature is well-known. Look it up and your questions will be answered.

 

[andrewbb]

Do you understand that for any material which is not at absolute 0 temperature the atoms are constantly moving? This is called thermal motion.

The higher the temperature of the material the faster the average velocity (higher KE) of the atoms, but there is always some statistical spread of velocities. The energy of individual atoms follows the Boltzmann distribution.

What is the cause of that motion? You say temperature means higher KE. That's a descriptive relationship, but specifically what is happening? I'm suggesting it is the electromagnetic charges repelling and attracting each other. Along with shape of the atom/molecule, it creates a rather random spread of velocities and trajectories.

 

[Char. Limit]

Andrew, you didn't listen. Of course two polar molecules will attract one another. But this isn't the PRIMARY reason for evaporation. Some problems with it (and this is all "at the molecular level"):

1. Water is polar, yes. But for the most part, air isn't (99% of air is nonpolar N2 and O2). How can a water molecule attach to a floating air particle when there is nothing to support it?

2. A water molecule is lighter than air, yes. But the only polar component of air that I can think of is... other water molecules. And that's about 1 part in 300. The attached water and "air" molecule would be heavier than the molecular mass of air.

3. Water molecules move at supersonic speeds. At room temperature.

 

[andrewbb]

Andrewbb: when water molecules stick together we call that ice. In liquid water, they do not stick together; they act more like regular atoms.

The bipolarity is just not that big of a factor, no matter how much you wish it so.

Stop hypothesizing.

The behaviour of water at room temperature is well-known. Look it up and your questions will be answered.

Dave, I HAVE looked it up. NO ONE describes it at a molecular level. They talk about general concepts of KE and temperature.

This isn't hypothesizing. This is thinking about the shape and properties of the water molecules and what is occurring as two or more water molecules collide.

 

[andrewbb]

Andrew, you didn't listen. Of course two polar molecules will attract one another. But this isn't the PRIMARY reason for evaporation. Some problems with it:

1. Water is polar, yes. But for the most part, air isn't (99% of air is nonpolar N2 and O2). How can a water molecule attach to a floating air particle when there is nothing to support it?

2. A water molecule is lighter than air, yes. But the only polar component of air that I can think of is... other water molecules. And that's about 1 part in 300. The attached water and "air" molecule would be heavier than the molecular mass of air.

3. Water molecules move at supersonic speeds. At room temperature.

I am listening.

Please show me where water molecules are moving at supersonic speeds. I don't see too many currents in the glass of water on my table.

 

[Char. Limit]

Wow, you can see the movement of particles whose size is measured in nanometers? Fractions of nanometers?

Water molecules are kind of small. Their motion is random, and as you get more and more of them (your water glass holds probably close to 10^25 water molecules) their random motion cancels itself out and tends to 0. In short, each water molecule is moving really fast, but they are all moving in random directions and cancel each other out, motion-wise.

 

[andrewbb]

Wow, you can see the movement of particles whose size is measured in nanometers? Fractions of nanometers?

Water molecules are kind of small. Their motion is random, and as you get more and more of them (your water glass holds probably close to 10^25 water molecules) their random motion cancels itself out and tends to 0. In short, each water molecule is moving really fast, but they are all moving in random directions and cancel each other out, motion-wise.

Do you have a reference on that?

If what you say is true, then certainly some of the molecules form hydrogen bonds with each other in a non symmetrical fashion. Those larger groups of molecules in hydrogen bond formation would create currents and not cancel each other out entirely.

 

[andrewbb]

I would also say that if the molecules are moving at supersonic speeds, their hydrogen bonds would be broken. That would break cohesion of water.

 

[Char. Limit]

Thus causing evaporation.

 

[Char. Limit]

If you mean having a reference on supersonic speeds... well, use the equation for the average (root-mean-square) speed of a water molecule:

\nu_{rms}=\sqrt{\frac{3kT}{m}}

m is the mass of one water molecule, in g, I think.
T is the temperature of the water molecule, in K.
k is Boltzmann's constant, which I don't have time to look up.

The left side is your molecular speed, measured in m/s. For reference, sound moves in room temperature air at 330 m/s.

 

[andrewbb]

What specifically is vibrating in the atom/molecule? As temperature increases, is the electron shell expanding? If so, then the electrical negativity charge on the atom would increase due to the proton(s) being of less influence. This would cause greater separation of the atoms/molecules at higher temperatures. Therefore the strength of electromagnetic charge is a function of temperature. Now, THAT makes sense in understanding thermal motion.

 

[andrewbb]

If you mean having a reference on supersonic speeds... well, use the equation for the average (root-mean-square) speed of a water molecule:

\nu_{rms}=\sqrt{\frac{3kT}{m}}

m is the mass of one water molecule, in g, I think.
T is the temperature of the water molecule, in K.
k is Boltzmann's constant, which I don't have time to look up.

The left side is your molecular speed, measured in m/s. For reference, sound moves in room temperature air at 330 m/s.

I think this is a misapplication of that equation. It applies to gases and I'm not sure it means actual velocity as much as potential velocity assuming nothing is in its path.

 

[andrewbb]

Interesting facts about evaporation of water in different fluids:

Butane evaporates water fastest
Helium evaporates water slowest

Butane's shape is a bunch of H's surrounding a few C's, leaving the H's free to attract O in water molecules. For those who still say I'm talking theory, take a look at "Lewis structures".

 

[DaveC426913]

This isn't hypothesizing. This is thinking about the shape and properties of the water molecules and what is occurring as two or more water molecules collide.

No, it is guessing. You do not know what happpens, so you are making it up.


What you are failing to grasp is that liquids comprised of polar molecules and liquids comprised of non-polar molecules behave essentially the same.

 

[Dale]

What is the cause of that motion?

Conservation of energy. Since the temperature is not absolute zero the atoms start out with some initial motion, then by conservation of energy the only way for one molecule to lose KE is to transfer it to another molecule in a collision. This then causes a spread, or distribution, of the number of molecules traveling at any particular speed, but the average energy remains constant. Billions of these collisions happen and, in the steady state, the distribution of the kinetic energies of the individual molecules is given by the Boltzmann distribution.

I'm suggesting it is the electromagnetic charges repelling and attracting each other. Along with shape of the atom/molecule, it creates a rather random spread of velocities and trajectories.

That is a nice suggestion, but the evidence does not back it up. The Boltzmann distribution can be measured in monatomic gasses which have neither a dipole charge nor a non-spherical shape, therefore charge and shape cannot be the reason for the observed distribution of speeds.

 

[Char. Limit]

Butane is a non-polar molecule. Here's another piece of info about butane:

There are two forms of butane, called n-butane, which is "slightly miscible" with water, and 2-methylpropane AKA isobutane, which is "immiscible" with water.

In other words, neither interacts with water much. A good thing to remember is that polar interacts with polar, and non-polar interacts with non-polar.

In other words, butane, which is non-polar (dipole moment = 0 D), won't interact with water, which is polar (dipole moment = 1.85 D).

Sorry.

 

[sponsoredwalk]

Intersting thread, I'm glad you keep asking questions but I think you don't understand some of the factors that are involved.

That explanation of evaporation is good, but doesn't describe the detail at the molecular level.

Water molecules are polar which means they are basically tiny magnets. The two hydrogen atoms sit on one side of the oxygen atom creating a positive charge on one side and negative on the other.

For a single molecule to evaporate it must overcome the cohesive force of the water (hydrogen bond). Hydrogen bonds are essentially magnetism. (The hydrogen is electromagnetically attracted to the oxygen in another molecule.)

You must misunderstand hydrogen bonding.

Oxegen atoms have a high electronegativity with a positive charge coming from it's 8 protons. When it forms a covalent bond with two Hydrogen atoms to form a water molecule, the attraction of all those protons "pulls" the electrons from the Hydrogen close towards it, resulting in a slight negative charge at this end of the water molecule. Conversely, the two Hydrogen molecules respective electrons are slightly displaced away from the single proton that "holds" them resulting in each H atom gaining a slight net positive charge.

This is the basis for Hydrogen bonding, the slightly positive hyrogen of one water molecule is attracted to the negative Oxygen of a neighbouring molecule. However, this hydrogen bond is about one-tenth as strong as a covalent bond (source: https://www.amazon.com/dp/0716798565/?tag=pfamazon01-20 Chap. 2&3). Also, the water molecules are continually breaking and re-forming hydrogen bonds due to their inherent motion. At anyone time a water molecule has around 3-4 hydrogen bonds formed but these quickly dissipate & reform with other molecules. This property is part of the reason why water has such a high boiling point, more heat energy is required to break these bonds.

While heat excites the molecules and churns them more vigorously, evaporation can happen in two ways:
1. two water molecules align so their oxygens repel from each other.
2. a water molecule hydrogen bonds to a molecule or atom in the air.

Does anyone have an argument to that way of looking at it? We are talking about the same thing, I am merely describing what happens to the individual water molecules. The temperature is a measurement of how vigorously the molecules are moving. If the molecules are moving around more vigorously, their hydrogen bonds are more likely to be broken which allows them to escape into the air.

Again, you assume that hydrogen bonds are like crazy magnetized forces. They are relatively weak but the huge accumulation of them is what gives water such a high boiling point. The molecular motion is too fast to sustain such bonds due to the kinetic energy of motion they have. Also, if you re-read that statement about molecules choosing the shortest path you'll see that most molecules will not interact in a mickey mouse shape and even if they do the natural repulsion of two molecules rebounding after "collision" would barely be influenced compared to the kinetic speeds.

When water freezes it is due to the molecules kinetic energy slowing to the point where more than the standard 3-4 hydrogen bonds can form, but they still break & re-form. To assume that's false is to say that atoms become stagnant when water freezes & that's wrong for so many reasons...

Water molecules are kind of small. Their motion is random, and as you get more and more of them (your water glass holds probably close to 10^25 water molecules) their random motion cancels itself out and tends to 0. In short, each water molecule is moving really fast, but they are all moving in random directions and cancel each other out, motion-wise.

This is the essence of Brownian Motion, [Read Einsteins paper on Brownian motion for more information - but this paper/reasoning is held as a proof for the existence of atoms], there are displacements on either side of a molecule at any particular time and this is "empty" space which is room for another molecule to fly into with kinetic energy to bounce another molecule. All throughout a glass of water they would roughly cancel out but at the top those molecules would fly out. There would be no water molecules of roughly equal kinetic energy to counteract this force. Again, it depends on environmental pressure & the kinetic energy of the particle, [I'll explain why environmental pressure is valid further down].

Furthermore, there wouldn't be as many hydrogen bonds forming with those molecules at the top of a glass of water. You can see that only the bottom half of the circumference of an imagined circular molecule is exposed to those water molecules underneath, the top half is exposed to the air molecules. Well, water molecules are the same.

idk the detail but let's assume half of the amount of hydrogen bonds are formed due to half of the surface area being exposed. Hydrogen bonding is still a weak force & the kinetic energy of even the odd "ice-water molecules" is enough to overcome this. I'll repeat, it's the accumulation of hydrogen bonding throughout a glass of water that gives it it's "strength", individual ones are pretty weak.

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid, or else the liquid would turn into vapor quickly (see boiling point). When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to escape.

Source: http://en.wikipedia.org/wiki/Evaporation

So, to be clear. If hydrogen bonds were such an important factor they would have to be stronger than they actually are & the fact that roughly half of the normal amount of Hydrogen bonds are capable of forming doesn't help.

Another really substantial claim in this thread which you didn't really address is why hydrophobic substances such as oil evaporate. How could hydrogen bonding explain how substances that share similar electronegativity to that of a water molecule (causing them to cluster when immersed in water & not interact with said water) evaporate due to hydrogen bonds when they simply do not form for these molecules?. You'll see below that atmospheric conditions (by these I mean, what's above the water - be it a vacuum or air) are important & contribute to all liquids that will evaporate.

I believe this is further proof to the molecular kinetic energy being the main factor. As Dave said earlier, Hydrogen bonds are merely a contributing factor... So these electromagnetic forces you keep mentioning are hydrogen bonds, well I hope you can put that idea to rest now. You also never mentioned anything like Van Der Walls forces, but I assume that's because you know they are far too weak to be of significance here.

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.[1][2] A liquid in a vacuum environment has a lower boiling point than when the liquid is at atmospheric pressure. A liquid in a high pressure environment has a higher boiling point than when the liquid is at atmospheric pressure. In other words, the boiling point of liquids varies with and depends upon the surrounding environmental pressure.

-----From http://en.wikipedia.org/wiki/Boiling_point

This quote from wikipedia gives insight to some of the claims made on this thread,

Air is irrelevant. You'll observe evaporation even in the vacuum.

&

1. Water is polar, yes. But for the most part, air isn't (99% of air is nonpolar N2 and O2). How can a water molecule attach to a floating air particle when there is nothing to support it?

2. A water molecule is lighter than air, yes. But the only polar component of air that I can think of is... other water molecules. And that's about 1 part in 300. The attached water and "air" molecule would be heavier than the molecular mass of air.

So, how could a weak polar bond form between the odd single water molecule in air and a water molecule in a glass moving with a good bit of energy? http://www.youtube.com/watch?v=zz4KbvF_X-0&feature=SeriesPlayList&p=166048DD75B05C0D This video gives you an idea of the energies the molecules have/need at specific temeratures. For a particle in the water to be "pulled" out by a single hydrogen bond with a H20 molecule in the air the force of attraction would have to be greater than the few Hydrogen bonds formed underneath this surface water molecule in the glass pulling it down, (if that's how evaporation occurred)...

I believe air pressure matters as the molecules of air above the glass would provide a repellant force down on these water molecules on the top of the glass flying up into them. Wouldn't that account for the temperature needing to be higher for these molecules to escape?

Wouldn't the water molecules require higher heat energy in order to gain a higher kinetic energy to overcome the downward force caused by the air above & the natural tendency for the water molecules to remain in the glass?

Doesn't that also show is why the temperature required for evaporation to occur in vacuo is lower?

What specifically is vibrating in the atom/molecule? As temperature increases, is the electron shell expanding? If so, then the electrical negativity charge on the atom would increase due to the proton(s) being of less influence. This would cause greater separation of the atoms/molecules at higher temperatures. Therefore the strength of electromagnetic charge is a function of temperature. Now, THAT makes sense in understanding thermal motion.

Do you understand electron shells, nevermind expanding ones...? If an electron shell were to expand wouldn't that mean the electrons would have to invent enough energy to overcome the attraction of the protons in it's nucleus, or to overcome the attraction of another atoms protons in a covalent bond? I'm not sure, hopefully somebody could answer that one for me...

Actually, the orientation is pretty much irrelvant. If two otherwise isolated water molecules collide with low enough kinetic energy to form a hydrogen bond then they will simply rotate into the minimum energy orientation regardless of their initial orientation. They are not constrained to always maintain a certain orientation.

Also, the dipole field of a water atom is electrostatic, not magnetic. If it were magnetic then water would jump onto a magnet.

That is a very interesting thing I didn't know, this minimum energy orientation thing reminds me of Snell's Law, the snippets I could unerstand from Introductory chapters on Lagrangian Dynamics & stuff Feynman wrote about in the sum over histories stuff (I vaguely recollect from Hawking book, or somewhere like that ). Basically choosing the shortest & easiest path. I hope to find out more about this in the future.

This is the way I see the process, if I've made an error please try to correct me constructively, and with a source if possible.

 

[andrewbb]

Thanks for that as it accurately describes what we are discussing. I am aware and agree with all you said.

Can you describe what's happening to a single molecule when you say "kinetic energy"? I know its relationship to temperature, but what is physically happening to that molecule? If you say "vibrating", what specifically is vibrating and why does temperature make it vibrate?

 

[sponsoredwalk]

http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html

Watch this animation. Specifically, when you see the ice structure and the atoms jiggling that is caused by the inherent kinetic energy of the molecules.

You understand what the word "Kinetic" means right? It means motion.

When you lift a football up off the ground, you are raising the footballs Potential Energy.

When you let go of the football it starts to fall down and increase in speed, releasing this Potential Energy in the form of Kinetic Energy, the energy of motion. Get a good introductory physics text to find out more of the details about it.

Here is a 30 minute lecture that you'd appreciate if you know a bit of math, but not essential to get the idea & enjoy the presentation.

http://video.google.com/videoplay?docid=-1865978801687874664#docid=-5313203621222966483

Also, the 14th lecture is worthwhile after for more or the math - good animations, (actually the whole 52 part series is worthwhile!).

Also, http://en.wikipedia.org/wiki/Temperature

Watch the two animations here and you'll see it perfectly. it is the exact same in water.
As for your last question, why does this temperature make the atoms move, you've phrazed that question wrong - at least in the way I understand this concept so far.

My understanding tells me that there is no such thing as a stationary atom, they are in perpetual motion. The temperature doesn't make the atoms move faster it merely records the fact that they are moving faster due to an increase in some form of energy.

I really can't be sure of this last part though & would appreciate somebody to clarify & expand upon that. Otherwise, I'll get back to you after all the thermodynamics texs I plan to study lol...

 

[andrewbb]

That's the thing. No one really has an explanation for WHY the atom is vibrating. They just call it "kinetic energy" and say it increases with temperature.


I think vibration of atoms is caused by increased electronegativity of the atoms/molecules. As temperature increases, the molecule's electronegativity increases. This weakens hydrogen bonds and repels molecules at a more vigorous rate. This seems obvious to me, but I have not seen anyone explain as such.

Now.. .why does the electronegativity increase with temperature? I think that's due to the proton's positive charges becoming of less effect. Possibly due to the electrons either becoming more active OR the electrons are moving away from the nucleus as temperature increases. I'd like to know the answer to this.

 

[DaveC426913]

No one really has an explanation for...

And you are sure of this because why?

I think...

You don't see a problem with this?

 

[Char. Limit]

I'm pretty sure, or so my Organic Chemistry textbook says, that what is vibrating in a molecule is the bonds themselves. They expand and contract, the angle of the bonds changes, and I think there's something else too.

In answer to your question to what vibrates in the molecule.