Are the gas laws compeletely true?

[russ_watters]

Volko, the first usage of "nonsense" was dealt with and over when you then said this:

Ideal gas law applies when water changes states? Who mentioned "nonsense" in this thread?

It was no less offensive (as was pointed out before you said it) for you to use it and making a point by asking a vague, rhetorical question is not helpful either.

Again, the bottom line is still that the ideal gas law is very useful in describing the atmosphere EVEN WHEN PHASE TRANSITIONS OCCUR.

Both the first usage of the word and yours were overzealous, but more to the point, your statement that the ideal gas law can't help at all with a phase transition is wrong.

 

[russ_watters]

Sample problems that can be solved primarily by using the ideal gas law (or derivative of it):

1.
1000 cubic feet of air enters an air conditioner at 90F and 50% RH and leaves at 55F. How much water vapor condensed into water?

2.
An air mass of 0.1 cubic miles volume at 80F and 50% RH blows over a large lake and is cooled to 70F by evaporating water. What is the new volume, RH and how much water evaporated from the lake?

Even with simplifications and shortcuts, both of these involve finding a mass of water vapor using absolute humidity. Absolute humdiity is calculated using the ideal gas law (even if you get it from a table, that's how it got into the table). See here:

Two basic laws apply to the air and vapor mixture that make our calculations possible. First, within the range of comfort air conditioning, the mixture follows the ideal gas laws...

Second, the gases follow Dalton's law of partial pressures.

http://siglercommercial.com/wp-content/uploads/2013/01/Psychrometric-TDP-Preview.pdf

So I would go so far to say that the ideal gas law is an essential part of atmospheric analysis for a wide variety of problems, if not most problems. In my line of work (heating and air conditioning), essentially all problems involving the thermodynamics of humid air utilize it.

 

[voko]

your statement that the ideal gas law can't help at all with a phase transition is wrong.

I fail to see how the ideal gas law can help with phase transitions. They do not manifest themselves in the law in any way. It was said by others, which I did not debate, that the residual gas is still described by the ideal gas law. But one needs something else, not the ideal gas law, to determine how much of that residual gas remains. I do not perceive that as "helping with a phase transition". It only helps with the residuals.

Or am I really missing something very big?

 

[russ_watters]

I fail to see how the ideal gas law can help with phase transitions. They do not manifest themselves in the law in any way. It was said by others, which I did not debate, that the residual gas is still described by the ideal gas law. But one needs something else, not the ideal gas law, to determine how much of that residual gas remains. I do not perceive that as "helping with a phase transition". It only helps with the residuals.

Or am I really missing something very big?

Did you see the sample problems I posted? For the first one, "How much of that residual gas remains" is precisely what the ideal gas law gives you when you plug in the new temperature and partial pressure.

 

[voko]

Did you see the sample problems I posted? For the first one, "How much of that residual gas remains" is precisely what the ideal gas law gives you when you plug in the new temperature and partial pressure.

Is the partial pressure given by the ideal gas law? I do not think so. You need something else to know it. This is what I have been saying all along.

 

[russ_watters]

Is the partial pressure given by the ideal gas law?

No.

You need something else to know it. This is what I have been saying all along.

No, your claims have gone far beyond that. There is a big difference between saying it is the only tool needed (which no one has claimed) and saying it can't be used at all (which is what you claimed).

 

[sophiecentaur]

56 Posts! and all because someone said you need to be cautious in practical circumstances.

 

[dauto]

Is the partial pressure given by the ideal gas law? I do not think so. You need something else to know it. This is what I have been saying all along.

Off course not. For finding the partial pressure you should use the Clausius–Clapeyron equation.
Where did you get the idea that I (or anybody else here) was saying you could solve a problem with just one equation? You may have to use more than one equation. My point is that the ideal gas law is one of those equations, and even if there is a phase transition, it will be accurate enough pretty much everywhere in the Earth's atmosphere, specially considering that the water vapor pressure is just a small component of the total atmospheric pressure. That may seem surprising since we all are supposed to have learned long ago that the ideal gas law does a poor job near transitions.

 

[dauto]

56 Posts! and all because someone said you need to be cautious in practical circumstances.

I think we've been talking past each other for most of those posts...

 

[K^2]

56 Posts! and all because someone said you need to be cautious in practical circumstances.

That's a trend around here. Almost every thread that should end with the first reply post being, "No," seems to run on for pages and pages.

 

[voko]

No, your claims have gone far beyond that. There is a big difference between saying it is the only tool needed (which no one has claimed) and saying it can't be used at all (which is what you claimed).

I have checked all my posts in this thread and I do not believe I ever said it could not be used at all. I explicitly agreed that it could be used for gas residuals. But not for the whole system, and some other law is required to find out the amount of the gas residue.

It seems to me that whether one can say the ideal gas law applies or does not in this situation is a matter of opinion. I have expressed my opinion, others have expressed theirs. If I have offended anyone by any figure of speech, I apologize. I have to excuse myself from further participation in this debate.

 

[russ_watters]

I have checked all my posts in this thread and I do not believe I ever said it could not be used at all. I explicitly agreed that it could be used for gas residuals. But not for the whole system, and some other law is required to find out the amount of the gas residue.

You said it could not be used at all when dealing with a phase transition. That's the scenario we are discussing:
"...the ideal gas law cannot describe a phase transition at all."

You repeated several iterations of that statement. What you are missing is that the ideal gas law doesn't have to describe the transition itself to provide an insight into what happened in the transition.

It seems to me that whether one can say the ideal gas law applies or does not in this situation is a matter of opinion.

No, it really isn't. Utilizing a particular equation to solve a problem will either give a right answer or a wrong answer (to a certain precision). I provided examples of problems that apply here and you ignored them.

There is very little in science that is a matter of opinion.

 

[klimatos]

Most gas laws require that conditions of equilibrium exist (if not throughout the process, then at least at the beginning and end of it). Since the Earth's atmosphere in never even close to a condition of equilibrium, one must be very, very cautious in trying to apply laboratory-valid gas laws to the free atmosphere.

I have been out of town for the last week, and never imagined the fuss that this simple caution would produce. I was referring to non-equilibrium conditions in the free atmosphere--not in the laboratory. The image I had in mind (but, unfortunately did not specify) was the violent and changeable winds accompanied by condensation that can be found in thousands of thunderstorms that are occurring even as you read this post.

If you have an equation of state that accurately relates ambient pressures (including wind pressures) to temperatures (both at the surface of the condensing droplet and in the air between) that is valid under these circumstances, I would dearly love to see it.

I am not unfamiliar with the ordinary gas laws (Boyle's Law, Charles Law, etc.), which is what I had in mind when I wrote the post. I have just reviewed them, and--sure enough--most of them require conditions of equilibrium to be valid.

What did I say that was so contentious?

 

[Chestermiller]

I have been out of town for the last week, and never imagined the fuss that this simple caution would produce. I was referring to non-equilibrium conditions in the free atmosphere--not in the laboratory. The image I had in mind (but, unfortunately did not specify) was the violent and changeable winds accompanied by condensation that can be found in thousands of thunderstorms that are occurring even as you read this post.

If you have an equation of state that accurately relates ambient pressures (including wind pressures) to temperatures (both at the surface of the condensing droplet and in the air between) that is valid under these circumstances, I would dearly love to see it.

I am not unfamiliar with the ordinary gas laws (Boyle's Law, Charles Law, etc.), which is what I had in mind when I wrote the post. I have just reviewed them, and--sure enough--most of them require conditions of equilibrium to be valid.

What did I say that was so contentious?

See the following reference:
Microphysics of Clouds and Precipitation (Atmospheric and Oceanographic Sciences Library) Paperback
by H.R. Pruppacher (Author) , J.D. Klett (Author)