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King E

Do molecules below the surface of the liquid evaporate?

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I heard, molecules only at the surface of the liquid evaporate. But do the molecules below the surface of the liquid evaporate? Suppose we heat a container containing a liquid from the bottom. So molecules at the bottom of the container will have higher kinetic energy than the molecules on the surface. How will the molecules on the bottom escape as vapours?

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36 minutes ago, King E said:

I heard, molecules only at the surface of the liquid evaporate. But do the molecules below the surface of the liquid evaporate? Suppose we heat a container containing a liquid from the bottom. So molecules at the bottom of the container will have higher kinetic energy than the molecules on the surface. How will the molecules on the bottom escape as vapours?

 

This question shows some good thinking.  +1

The answer is molecules do both, but under different circumstances, notably the heating you have introduced.

The condition where molecules escape principally from the surface is an equilibrium condition, where the temperature of the vapour and liquid are the same.

Adding heat allows the liquid temperature to exceed that of the vapour, to the extent that the liquid eventually boils and internal surfaces (bubbles) form.

This can all be represented on a phase diagram.

How complicated do you want to get?

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4 hours ago, King E said:

How will the molecules on the bottom escape as vapours?

To give an even simpler answer: they get to the surface and then escape! 

That may be too simple to be useful. But as you add heat from the bottom, it will cause convection which brings those molecules up to the surface. Also, as the fastest molecules at the surface escape, the average kinetic energy of the top layer decreases; in other words it cools and so will sink lower in the container.

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Posted (edited)
On 8/22/2020 at 6:54 PM, Strange said:

To give an even simpler answer: they get to the surface and then escape! 

That may be too simple to be useful. But as you add heat from the bottom, it will cause convection which brings those molecules up to the surface. Also, as the fastest molecules at the surface escape, the average kinetic energy of the top layer decreases; in other words it cools and so will sink lower in the container.

Why doesn’t that happen by conduction, when the molecules below vibrate upon heating and collide with each other they transfer energy all the way through other molecules to the surface molecules?

Edited by King E

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7 hours ago, King E said:

Why doesn’t that happen by conduction, when the molecules below vibrate upon heating and collide with each other they transfer energy all the way through other molecules to the surface molecules?

Conduction will take place as well. But, in a liquid, heat transfer by convection will be much faster.

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I would say that molecules NEVER "evaporate".  It is the body of the water that evaporates as molecules leave the body of the water.  Of course when you heat water to its boiling point,  bubbles will appear in the water.   Do you want to count that as "evaporation"?

 

 

 

 

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1 minute ago, HallsofIvy said:

I would say that molecules NEVER "evaporate".  It is the body of the water that evaporates as molecules leave the body of the water.  Of course when you heat water to its boiling point,  bubbles will appear in the water.   Do you want to count that as "evaporation"?

Depends, who wants it most?

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9 hours ago, King E said:

when the molecules below vibrate upon heating and collide with each other

 

Molecules in a liquid don't vibrate.

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4 hours ago, studiot said:

 

Molecules in a liquid don't vibrate.

Yes they do.
 

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1 minute ago, John Cuthber said:

Yes they do.
 

Do tell.

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4 hours ago, studiot said:

Molecules in a liquid don't vibrate.

You must have a very narrow definition of "vibrate".

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Posted (edited)
26 minutes ago, Strange said:

You must have a very narrow definition of "vibrate".

Not at all.

Molecules (in liquids) dance, they spin around, they waggle and wiggle, but they (the molecules) do not vibrate.

Molecules in solids can vibrate.

Individual parts of molecules (in liquids) can vibrate relative to other individual parts, but the molecule as a whole does not vibrate.

We are only interested in motion that can lead to translation (ie evaporation) and that is the first one on my list.

To vibrate requires a fixed restoring force, which is present in solids, but not in liquids.

Such a restoring force is present within a molecule, but relative displacement of parts of an object cannot lead to displacement of the whole object in isolation.

Edited by studiot

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Better tell all these people, then.

Quote

Publishing in Science, EPFL scientists have used lasers to determine for the first time how specific vibrations in a water molecule affect its ability to dissociate.

https://phys.org/news/2014-05-molecular-vibrations-hydrogen.html

Quote

Breaking up water: Controlling molecular vibrations to produce hydrogen

https://www.sciencedaily.com/releases/2014/05/140501142227.htm

Quote

Water molecules contain three atoms and so can vibrate in a number of different ways. 

http://www.schoolphysics.co.uk/age16-19/Wave properties/Wave properties/text/Microwave_ovens/index.html

Quote

Innovative Microscopy Visualizes Vibration of Water Molecules Trapped at Nanoscale

https://www.azonano.com/news.aspx?newsID=36282

Quote

In addition to rotational and translational motion, the atoms within a molecule also undergo an oscillatory motion.

http://www.wiredchemist.com/chemistry/instructional/supplemental-material-for-chemistry/chapter-7/animations

That last one also describes the most obvious form of vibration, which is also temperature dependent and so, presumably, what the OP was thinking of:

Quote

Molecules in the gaseous and liquid states are in constant motion, translating in straight lines until they collide with another molecule or the walls of the container. 

 

Now, try saying that those are not "vibration" at the same time as saying that your definition is "not narrow".

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9 minutes ago, Strange said:

dissociate.

 

9 minutes ago, Strange said:

Breaking up water

 

 

19 minutes ago, Strange said:

Water molecules contain three atoms and so can vibrate in a number of different ways. 

 

How are these a vibration of the molecule as a whole?

41 minutes ago, studiot said:

, but the molecule as a whole does not vibrate

 

11 minutes ago, Strange said:

Molecules in the gaseous and liquid states are in constant motion, translating in straight lines until they collide with another molecule or the walls of the container. 

Indeed so, did I not say say that the key motion is translation of the whole molecule ?

How is this a vibration?

 

 

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25 minutes ago, studiot said:

How is this a vibration?

Obviously, it isn't in your narrow definition.

"No true Scotsman ..."

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52 minutes ago, studiot said:

Not at all.

Molecules (in liquids) dance, they spin around, they waggle and wiggle, but they (the molecules) do not vibrate.

Molecules in solids can vibrate.

Individual parts of molecules (in liquids) can vibrate relative to other individual parts, but the molecule as a whole does not vibrate.

We are only interested in motion that can lead to translation (ie evaporation) and that is the first one on my list.

To vibrate requires a fixed restoring force, which is present in solids, but not in liquids.

Such a restoring force is present within a molecule, but relative displacement of parts of an object cannot lead to displacement of the whole object in isolation.

Mainly wrong.
For a start
https://en.wikipedia.org/wiki/Infrared_spectroscopy#Liquid_samples

Also, re" We are only interested in motion that can lead to translation (ie evaporation) and that is the first one on my list."
No
If you look at a simple molecule like water you can calculate (quite easily) the number of possible vibrational modes it has.
Each atom can move in any of 3 directions, (x,y and z).
There are 3 atoms in a water molecule.

So there are 3X3= 9 possible ways in which the atoms can move. But three of those correspond to rotations  while (and here's the important bit) three of them correspond to translations.
So there can be no more than 3 vibrational modes for water- it absorbs (fundamentally) at just 3 wavelengths.

https://en.wikipedia.org/wiki/Infrared_spectroscopy#Number_of_vibrational_modes
So the movements that you are interested in- the translations- are definitely not vibrations.

 

(Just in case you are wondering, for linear molecules where rotating about the axis of the molecule isn't defined, there  is one less rotational motion and thus 3n-5 possible vibrations.)

1 hour ago, studiot said:

To vibrate requires a fixed restoring force, which is present in solids, but not in liquids.

Actually, what you want is a variable restoring force- if it varies linearly with distance you get a sinusoidal vibration.

Molecules have bonds.
Those bonds behave (classically) as if they're springs.

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Quote

According to the theory, the temperature of a substance is proportional to the average kinetic energy with which the molecules of the substance are moving or vibrating. 

https://www.britannica.com/science/Brownian-motion

And I always thought the Encyclopaedia Brittanica was such a reliable source of information. But apparently, I should take the word of some RGOTI instead.

(About 1/3rd of the search results for Brownian motion also include the word vibration)

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12 minutes ago, Strange said:

But apparently, I should take the word of some RGOTI instead.

Yep.

Just make sure it's a randomly chosen spectroscopist.

 

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Seems John and Strange are considering intramolecular vibrations, otherwise known as stretching, bending and twisting.
Studiot's intermolecular vibration is considering only harmonic motion of the CoM of the molecule; and that does require a restoring force.
A narrow definition, to be sure, but he did qualify it.

59 minutes ago, studiot said:

Indeed so, did I not say say that the key motion is translation of the whole molecule ?

How is this a vibration?

 

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7 minutes ago, MigL said:

Seems John and Strange are considering intramolecular vibrations, otherwise known as stretching, bending and twisting.

Not just that. Most people would describe the random motions of water molecules as vibration (see citation above). Maybe that is a broader definition of the word than some would like, but it is clearly the one the OP was thinking of.

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I would call random motions of the whole molecule, 'Brownian'; not vibrational.
Darn definitions !

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1 minute ago, MigL said:

I would call random motions of the whole molecule, 'Brownian'; not vibrational.
Darn definitions !

Brownian motion is what the random vibrations of the molecules impart to larger particles!

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Now you are being narrow in your definition.
Molecules also collide among themselves, with resultant 'Brownian' motion of the  impacted molecule.

IOW, if you can identify a single water molecule in the liquid, with a radioactive marker, say O15, you will see it do the random walk of Brownian' motion.
 

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8 hours ago, MigL said:

Now you are being narrow in your definition.

And those particles have to be brown.

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