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Offline neilep

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Boiling water on the moon !
« on: 04/09/2007 07:10:38 »
Hi there,

here's some hot water...




careful now !!...


Let me get this right....the higher the altitude...the less pressure there is on the water and therefore the quicker it boils and the lower the temperature it boils at too yes ?

So, what temperature will my pot boil water at on the Moon ?


 

Offline Soul Surfer

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Boiling water on the moon !
« Reply #1 on: 04/09/2007 08:54:10 »
At a tiny bit above the normal freezing point of water known as the triple point of water becasuse (assuming the water is initially at room temperature) the water will start to boil and cool the remaining water until it frezes there will eventually be a mix of liquid water and ice which will evaporate slowly depending on how much heat it receives from radiation if it is in the sun it will evaporate quite quickly but if it is dark it will evaporate very slowly.
 

Offline Bored chemist

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Boiling water on the moon !
« Reply #2 on: 04/09/2007 21:50:09 »
The water will boil imediately and the temperature will drop to the temperature where the vapour pressure of ice equals the ambient pressure (ie not a lot). Of course, if the water was very hot to start with it might all boil away before the temperature droped that far.
The triple point is only relevant if you have the right pressure and that's a few mmHg not the near perfect vacuum of the moon.
« Last Edit: 04/09/2007 21:58:04 by Bored chemist »
 

Offline DrDick

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Boiling water on the moon !
« Reply #3 on: 07/09/2007 04:18:43 »
The more I think about this question, the more complicated it becomes.

Liquid water wouldn't exist on the moon under equilibrium conditions.  At the moon's normal atmospheric pressure (almost nonexistent), water exists only as a gas or solid, so it sublimes.  However, given that there is no atmosphere to speak of, there is no wind, so the gaseous water would hang around the solid to some degree, raising the local atmospheric pressure to a potentially measurable quantity.  In order to melt, the local atmospheric pressure would have to rise to more than 6.1 mbar (0.0060 atm).  I think gaseous diffusion processes would be enough to prevent this, but I'm not certain. 

It would certainly depend on whether it was day or night, since diffusion is dependent on the kinetic energy of the molecules, which would be heavily dependent on the amount of solar radiation being received.

So, if you took a pot of liquid water from your lunar lander, it would immediately start to boil, because the temperature of the water would be above the phase change temperature.  Boiling is an endothermic process, however, so the temperature of the water would cool fairly rapidly until it started to freeze.  At this point, the water would be freezing and boiling at the same time, until you only had solid water left in the pot.  This would then gradually sublime away until your pot was empty.

Dick
 

Offline DrDick

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Boiling water on the moon !
« Reply #4 on: 07/09/2007 04:21:21 »
Well, I guess I ended up saying the same thing that BC said, although it wasn't what I had in mind when I started my post.   [:I]

Dick
 

Offline Soul Surfer

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Boiling water on the moon !
« Reply #5 on: 07/09/2007 09:42:25 »
Chemist I agree I was wrong the triple point pressure is pretty low but not as low as the space around the moon it would boil away until it all froze solid and would cool down to the local temeperature which is very cold if it is in the shade.  If it was in the sun it would still freeze and get pretty cold until it all evaporated.

If the water was initially at boiling point  evaportation takes about 500 calories per gram and taking water from boiling to frreezing requires about 100 calories per gram and freezing it about 80 calories so more than half the boiling water would be left when it froze.
 

Offline DoctorBeaver

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Boiling water on the moon !
« Reply #6 on: 09/09/2007 08:40:34 »
So, basically, what you're all saying is that if I want a cup of tea, don't go to the moon :(
 

Offline lightarrow

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Boiling water on the moon !
« Reply #7 on: 09/09/2007 12:59:21 »
So, basically, what you're all saying is that if I want a cup of tea, don't go to the moon :(
Not even if you want to stay integer: *you* also , will boil immediately there (I mean, all your blood and the body fluids). I assume you have never seen the film "Atmosphere zero".
 

Offline Bored chemist

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Boiling water on the moon !
« Reply #8 on: 09/09/2007 13:37:47 »
The mean suface temperature of the moon is over 100C so it's hot enough to make tea. There's no question that there's enough heat ther in most places to boil off water. Tucked away in craters near the poles there may be places that never see the sun. These might have ice in them but even this sill sublime away slowly.
An ordinary pressure cooker is designed to operate at a pressure about 1 atmosphere above ambient so it should be possible to use one on the moon to make boiling water at about the right temperature for tea.
 

Offline DrDick

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Boiling water on the moon !
« Reply #9 on: 10/09/2007 16:29:03 »
The mean suface temperature of the moon is over 100C so it's hot enough to make tea. There's no question that there's enough heat ther in most places to boil off water. Tucked away in craters near the poles there may be places that never see the sun. These might have ice in them but even this sill sublime away slowly.
An ordinary pressure cooker is designed to operate at a pressure about 1 atmosphere above ambient so it should be possible to use one on the moon to make boiling water at about the right temperature for tea.

The temperature may be high on the day side of the moon, but that doesn't necessarily mean there's enough heat around to boil water.  With no atmosphere to speak of, the only sources of heat are direct solar irradiation and the ground (once it's been heated by the sun).  Remember that the heat capacity of the lunar atmosphere is essentially zero.

Dick
 

Offline Bored chemist

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Boiling water on the moon !
« Reply #10 on: 10/09/2007 20:01:28 »
Imagine some ice exposed on the surface. It's very cold but, thanks to the random nature of the Boltzman type distribution, a few molecules of water will happen to gain enough energy to drift off into space. There's essentially no chance of it coming back. This means that the mean energy of the ice molecules left behind is slightly reduced ie that they are colder. If the stuff was at equilibrium before then it isn't any more. Heat will flow into it (slowly) from its surroundings to bring it back to the temperature it was before.

Repeat until the ice is gone.
 

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Boiling water on the moon !
« Reply #10 on: 10/09/2007 20:01:28 »

 

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