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.