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Some Scientific Problems (Geophysics)
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Chad


In: Ramsbottom
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Let me put it another way. Is this a "model" that physicists tell one another or is it an observed fact?

Don't know what physicists tell one another... But to improve my analogue further, replace eider down with ping-pong balls and employ a more powerful fan....one of them big jobbies they use to shift leaves oughta do it. (Leave net in place).
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Mick Harper
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So, a model. Thought so.

PS Brian, what's the difference between being completely saturated and being water. From a molecule's point of view.

PPS In other words, what's stopping it becoming so.
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Brian Ambrose



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PS Brian, what's the difference between being completely saturated and being water. From a molecule's point of view.


Being water means all your molecular neighbours are (essentially) water molecules.
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Chad


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Right then.... To look at this properly we need to get rid of the air (all that nitrogen and other crap) so all we have is liquid water and water vapour.

For that we need an uncollapsible sealed container (partly filled with water) and a very powerful vacuum pump.

Now things look a bit more interesting....
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Brian Ambrose



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Yes, that's much better.

The water molecules in gas form have sufficient kinetic energy (and enough space) to avoid clumping together (ie overcome their mutual attraction). Input energy to increase the density of the gas, remove energy to decrease it.
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Chad


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So if we start with a given volume of pure liquid water (no dissolved gasses) and gradually increase the volume available to that given mass of water (without allowing in any other molecules) it will at some point change en masse from being a liquid to being a gas.

There can be no stable point at which the vessel contains both liquid and gaseous water at the same time... or can there?
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Chad


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Input energy to increase the density of the gas, remove energy to decrease it.

Wouldn't the amount of energy input simply affect pressure?

Wouldn't you need to alter the volume to change density?

Arr... unless you are talking about something less that an absolute perfect vacuum... where almost pure water vapour could coexist in the space above the liquid water... then putting energy in would release more molecules from the liquid and increase the density of the gas.
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Brian Ambrose



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Chad wrote:
There can be no stable point at which the vessel contains both liquid and gaseous water at the same time... or can there?


Don't see why not. If the net energy input is zero (ie losses=input) there ought to be a stable point at which the gas pressure prevents further evaporation (ie where an equal number of gas molecules randomly exchange state with liquid ones).

Even with a net energy input the amount of liquid which can be converted to gas will depend on the volume of the container and the volume of original liquid (ie consider the extreme case where vol. water=container vol).
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Mick Harper
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So if we take a closed system which starts off as part water and part air, leave it for the requisite time, then return to examine it we must end up with a closed system that is part liquid water and part saturated air. Irrespective of what the molecules are doing hopping between the two states, the overall situation will be in equilibrium with water at the bottom and the rest saturated air. Is this correct?
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Chad


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Brian Ambrose wrote:
Chad wrote:
There can be no stable point at which the vessel contains both liquid and gaseous water at the same time... or can there?


Don't see why not...

If you start with a totally sealed container, absolutely full with nothing but 100% pure liquid water, at atmospheric pressure, then expand the volume... the reduction in pressure will gradually cause the liquid to expand to take up the new volume, until the molecular bonds can no longer retain liquid form. At that point the liquid will spontaneously boil and will en masse start to turn into a gas.

You can't have both states existing (in stable form) at the same time.
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Brian Ambrose



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At that point the liquid will spontaneously boil and will en masse start to turn into a gas.


en masse? No, just sufficient to occupy the extra volume (ie the gas creates a balancing pressure).
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Brian Ambrose



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Mick Harper wrote:
So if we take a closed system which starts off as part water and part air, leave it for the requisite time, then return to examine it we must end up with a closed system that is part liquid water and part saturated air. Irrespective of what the molecules are doing hopping between the two states, the overall situation will be in equilibrium with water at the bottom and the rest saturated air. Is this correct?


With the proviso that I don't really know what 'saturated air' is, I would assume that under certain conditions (eg temperature) you would reach a state of equilibrium, yes. But I don't think Chad agrees.
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Chad


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Brian Ambrose wrote:
With the proviso that I don't really know what 'saturated air' is, I would assume that under certain conditions (eg temperature) you would reach a state of equilibrium, yes. But I don't think Chad agrees.

No, no... I agree entirely.

The condition I was trying to describe occurs exclusively when the only molecules present are pure water. When air is present the gaseous water molecules just take their place among the other airborne molecules.

But when the only molecules present are pure water, whatever change in pressure is applied (due to change in volume) affects all the molecules equally... so how can some decide to become gas while others remain liquid.

No... It's one for all and all for one.
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Mick Harper
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So, both our scientific consultants, Chas and Bri, agree that in a closed system, given enough time, all that will be left for inspection is a) liquid water at the bottom and b) saturated air at the top.

Since we can agree that a billion years is long enough the only thing remaining is to confirm that the earth is a 'closed system' within the meaning of the act. The ocean has nowhere to go, it doesn's (it would seem and in any great quantities) seep into the crust or anything. The atmosphere has nowhere else to go either unless somebody wants to argue that water vapour is boiling off into space. This would be highly revisionist in itself but presumably if true everything would have boiled away slightly less than a billion years ago.

So we have a closed system. The water at the bottom is there all right but where is the saturated atmosphere? It is nowhere near saturated nor hardly ever gets that way. Explanations, please.
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Brian Ambrose



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But when the only molecules present are pure water, whatever change in pressure is applied (due to change in volume) affects all the molecules equally... so how can some decide to become gas while others remain liquid.


1. Because the change in pressure does not affect all the molecules equally. The molecules at the bottom of the tank have no notion of the conditions felt at the surface layer.

2. Even if they did (say, there was no gravity), there is only so much room for gas when the volume is expanded. As I said previously, there is negative feedback in the system. The vacuum which you rely on is no longer a vacuum.

It cannot be right that a container full of water could be vapourised simply by making the container 1% bigger - unless you maintain the vacuum by removing the gas.
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