Water vapors in air

  • #1
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Main Question or Discussion Point

While replying to my question(s) please keep in mind that I'm not a math or science student - quite a layman in these walks. So, please explain your answer in as much detail as possible. Thank you.

Placing your hand over a boiling water could burn your hand. That means water vapours' temperature is higher than the surrounding area - I don't know if it's 60C, or 100C. 100C is given as water's boiling point which doesn't really tell how much kinetic energy vapours have on Kelvin scale.

Suppose a room's temperature is 25C. There are also water vapours in the air which are, I'm sure, above 50C, at least. This would mean constituent particles of air are at temperatures throughout and that room's temperature, 25C, is just an average.

Do I make sense? Please guide me.
 

Answers and Replies

  • #2
Welcome to the wonderful world of phase diagrams !

Water vapour is present in air at any temperature above about 0deg C (assuming atmospheric pressure).
Above 100deg C all the water is as a vapour.

The water vapour above the kettle is the same as the water vapour in the air - just a lot hotter
 
  • #3
Borek
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Water vapour is present in air at any temperature above about 0deg C (assuming atmospheric pressure).
No, water vapor is present in the air always.

Welcome to the wonderful world of phase diagrams :tongue2:
 
  • #4
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Thank you, NBSpecial, Borek.

I'm sorry to say but I think you have missed the main question. Let me rephrase it:

This would mean constituent particles of air are at different temperatures (highly diverse kinetic energies) throughout and that room's temperature, 25C, stand for only average temperature, average kinetic energy.

What minimum kinetic energy a water molecule should have in order to stay in air at any particular temperature?

And what this all welcome about phase diagrams?! :-)
 
  • #5
No, water vapor is present in the air always.
The triple point is the lowest temp that all three can exist together isn't it?
So I thought that (at least at the same pressure) you couldn't have vapour below this ?

Welcome to the wonderful world of phase diagrams :tongue2:
One of the many reasons, along with a tendancy to bite my fingernails, that I didn't become a chemist!
 
  • #6
This would mean constituent particles of air are at different temperatures (highly diverse kinetic energies) throughout and that room's temperature, 25C, stand for only average temperature, average kinetic energy.
Yes - ultimately they will all come to the same average temperature and so the same kinetic energy. But if you have a boiling kettle in one part of the room and a block of ice in another there will be different air temperatures and so different kinetic energies


What minimum kinetic energy a water molecule should have in order to stay in air at any particular temperature?
Thats what the phase diagram tells you. It's a map of at what combination of temperature and pressure you can have solid, liquid or gas.

They look something like
512px-Water_phase_diagram.svg.png
 
  • #7
This would mean constituent particles of air are at different temperatures (highly diverse kinetic energies) throughout and that room's temperature, 25C, stand for only average temperature, average kinetic energy. What minimum kinetic energy a water molecule should have in order to stay in air at any particular temperature?
The temperature of the air is directly related to the average energy of the air particles. There is no strict minimum or maximum energy for a given particle. Just imagine a bell-shaped curve where you have the number particles on the vertical axis and kinetic energy on the horizontal axis.

As soon as a water molecules escapes the boiling water, because of its excess energy, it starts cooling by giving away it's energy to the surrounding air molecules. IT cools off until it's own temperature has reached 25degC, but by then it has moved away from the pot quite a bit. So the average temperature of the air+vapor around the pot decreases as the distance from the pot increases.

Unless I misinterpret your question, I don't think you need to bother with phase diagrams at this stage.
 
  • #8
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Yes, Dr Lots, phase diagrams aren't needed at this stage. I like to learn science from a 'physical' point of view and not as a bunch of theories and formulae. Thanks a lot.

I was wondering why it is so that as air cools down the quantity of water vapors decreases. What is 'physical' interpretation of this?

Please guide me.

Thank you, everyone.
 
  • #9
russ_watters
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I was wondering why it is so that as air cools down the quantity of water vapors decreases.
The air is largely irrelevant except that it makes the water vapor come to the same temperature. The issue is simply that the boiling point of water is lower at lower pressure because the pressure of the water vapor above the water has an easier time holding the water molecules together in liquid form when the liquid water molecules aren't bouncing around as much.
 
  • #10
russ_watters
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Welcome to the wonderful world of phase diagrams !

Water vapour is present in air at any temperature above about 0deg C (assuming atmospheric pressure).
Above 100deg C all the water is as a vapour.

The water vapour above the kettle is the same as the water vapour in the air - just a lot hotter
There's a little more to it than that: 100C/212F air blown across your hand will not burn it, but 100C/212F water vapor will.

Just above the kettle, you have essentially 100% water vapor and no air. When the water vapor hits your hand, the temperature of your hand is lower and causes a portion of the water vapor to condense into liquid water. The act of condensation releases a large quantity of energy, which causes the burn.
 
  • #11
Borek
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The triple point is the lowest temp that all three can exist together isn't it?
So I thought that (at least at the same pressure) you couldn't have vapour below this ?
Take a look at phase diagram and check which phases will be not present at 1 atm and -10 °C (which is what I - more or less - see through the window right now).

And no, triple point is not the lowest temp that all three can exist, it is the only combination of temp and pressure where they all exist at the same time.
 
  • #12
Borek
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Yes, Dr Lots, phase diagrams aren't needed at this stage. I like to learn science from a 'physical' point of view and not as a bunch of theories and formulae.
Mission impossible. You can get some basic understanding, but with this approach you are severely limited.
 
  • #13
sophiecentaur
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I don't think anyone has yet, in this thread, mentioned the 'real reason; for the fact that holding your hand over boiling water makes it feel hot.
When your hand is in contact with a hot gas (air), your skin will reach an equilibrium temperature such that the heat transferred into your body balances the heat transferred from the gas. The 'heat carrying property' of the gas in contact with your skin is much less than that of your tissue and blood circulation, so the temperature it arrives at will be 'comfortable'.
When boiling liquid water is changed into vapour, heat needs to be supplied. This is referred to as Latent Heat, because the actual temperature of the water doesn't go above 100C. When this vapour condenses on your hand, it releases this Latent Heat and supplies a lot of heat right there onto your skin - much more than you would get from air of the same temperature. It raises the local temperature of your skin way above what it would attain (thermal equilibrium) in the presence of just 'hot air'. Its 'heat carrying property' is much higher so it 'feels' hotter.
 
  • #14
russ_watters
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I didn't say "latent heat", but I described it in post #10.
 
  • #15
And no, triple point is not the lowest temp that all three can exist, it is the only combination of temp and pressure where they all exist at the same time.
So below freezing you can have ice and vapour but not liquid - hadn't thought of it like that.
 
  • #16
sophiecentaur
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I didn't say "latent heat", but I described it in post #10.
Yes - fair eeenufff.
I searched for the word "latent" in addition to scanning through but missed your wise words, Russ.
But the general message of the thread actually ignores the OP question and I think that the main mechanism is, in fact, Latent Heat. It's a fantastic way of transferring heat - the very biggest transmitting valves use 'vapour phase cooling' which is a lot better value than just pumping water round the cooling circuit. I wonder why auto engines don't use the same thing. But, there again, Heat Pipes do it that way, too, I suppose.
A great example of this effect is to pick up a hot pan with a damp cloth rather than with a dry one!!! Owch.
 
  • #17
It's a fantastic way of transferring heat - the very biggest transmitting valves use 'vapour phase cooling' which is a lot better value than just pumping water round the cooling circuit. I wonder why auto engines don't use the same thing
Liquid is simpler and safer - in an auto engine you aren't short of space and you don't have very high power densities. You also have a large thermal mass of engine block to provide some initial heat reservoir while the water starts flowing.

CPUs can dump >100W in 1cm^2 so need some more efficent cooling system such as heat pipes.
 

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