ramsharma Posted July 6, 2007 Share Posted July 6, 2007 Hi, As I was reading the gas laws - a thought came by, would liquid water change to gaseous form by reducing the pressure only. if yes, at what pressure would this happen - assuming temp is kept constant. Ram Sharma bvgsrs@yahoo.com Link to comment Share on other sites More sharing options...
insane_alien Posted July 6, 2007 Share Posted July 6, 2007 yes it would happen, and the pressure it happens at depends on the temperature. or, more specifically, the vapour pressure which is proportional to temperature. things boil when the vapour pressure is the same as the pressure of the system. so you need to lower the pressure ofthe system till this pressure is reached. off the top of my head, it is 4.2 kPa at room temperature Link to comment Share on other sites More sharing options...
swansont Posted July 6, 2007 Share Posted July 6, 2007 This is a standard physics demonstration, since this is several percent of atmosphere and is easily attainable. Mechanical pump, or possibly even a hand pump, and a bell jar. (But i_a must like it warmer than I do, since that's the vapor pressure at 30 C. At 20 C it's 2.3 kPa.) http://www.fordhamprep.org/gcurran/sho/sho/reference/table74a.htm Link to comment Share on other sites More sharing options...
insane_alien Posted July 6, 2007 Share Posted July 6, 2007 well, i was in the right ball park. and anyway, i pulled that number from memory because i couldn't be bothered reaching over for my book of steam tables Link to comment Share on other sites More sharing options...
igosaur Posted July 8, 2007 Share Posted July 8, 2007 Presumably, this is why there are no lakes or rivers left on Mars. Since the atmosphere became stripped away by the Solar winds, the pressure dropped and hey presto. What didn't freeze into the surface simple evaporated away. Hi, As I was reading the gas laws - a thought came by, would liquid water change to gaseous form by reducing the pressure only. if yes, at what pressure would this happen - assuming temp is kept constant. Ram Sharma bvgsrs@yahoo.com Link to comment Share on other sites More sharing options...
YT2095 Posted July 8, 2007 Share Posted July 8, 2007 Mars has an atmosphere, it`s just toxic. Link to comment Share on other sites More sharing options...
igosaur Posted July 8, 2007 Share Posted July 8, 2007 This is very true. However, the pressure is so low that if you were to place a bowl of water on its surface it would boil/evaporate away. Mars has an atmosphere, it`s just toxic. Link to comment Share on other sites More sharing options...
insane_alien Posted July 8, 2007 Share Posted July 8, 2007 actually, the martian surface has a maximum temperature of -5 *C with a pressure of 700-900 Pa at 0*C water has a vapour pressure of 611 Pa. it would not boil but it would freeze. it would have to get to 2*C to boil. surface pressures and temperatures taken from wikipedia. vapour pressures of water taken from Thermodynamic and Transport Properties of Fluids, 5th edition, G.F.C. Rogers and Y.R. Mayhew. Link to comment Share on other sites More sharing options...
igosaur Posted July 8, 2007 Share Posted July 8, 2007 I stand corrected yet again Let's say that on a warm sunny martian day it would boil away, or, it was a bowl of warm water it would evaporate before it froze. Link to comment Share on other sites More sharing options...
swansont Posted July 8, 2007 Share Posted July 8, 2007 However, it can evaporate or sublimate while below the boiling point. Link to comment Share on other sites More sharing options...
insane_alien Posted July 8, 2007 Share Posted July 8, 2007 no, some would evapourate and the rest would freeze. you can try this yourself, you'll need a pressure cooker, a stack of bricks a small fire, and a long stick. fill the pressure cooker with water and stack bricks on top instead of using the clamps.then put it on the fire(do this outside). wait until the release valve starts letting out steam. this means the temperature is around 120*C and the vapour pressure is 2 bar. (now, the dangerous bit, try doing it from behind cover) using the stick, push the bricks up, the lid will fly off and probably a brick or two so watch out. there will be a great woosh of steam as a lot of it flash boils. after this has happened and the lid has landed, go look in the pot, there will still be a good amount of water left. Link to comment Share on other sites More sharing options...
igosaur Posted July 8, 2007 Share Posted July 8, 2007 And then the water that's left, using the Martian example, would then freeze. But then what about the water vapour in the thin atmosphere, would that then rain back as a fine frozen mist I wonder? I wish I could go there and find out Link to comment Share on other sites More sharing options...
insane_alien Posted July 8, 2007 Share Posted July 8, 2007 well, there is very little water vapour in the martian atmosphere, the water vapour formed would just disperse. there would be little or no precipitation whatsoever. Link to comment Share on other sites More sharing options...
igosaur Posted July 8, 2007 Share Posted July 8, 2007 Therefore raising the atmospheric pressure of Mars ever so slightly thus increasing the temperature needed for water to evaporate. Link to comment Share on other sites More sharing options...
insane_alien Posted July 8, 2007 Share Posted July 8, 2007 no, the pressure would not noticably increase from a bowl of water left out. Link to comment Share on other sites More sharing options...
YT2095 Posted July 8, 2007 Share Posted July 8, 2007 lets see if I`ve got this right, you say 700-900 Pa, so if we be generous and make it 1000 Pa (or 1 KPa) and here on Earth 101KPa is standard room pressure, then it`s well over 1/100`th of Earths pressure on Mars? correct? Link to comment Share on other sites More sharing options...
igosaur Posted July 8, 2007 Share Posted July 8, 2007 Even though it would be impossible to measure, the pressure must increase to some degree, even though it may just be a local effect. After all, if you left enough bowls of water out, at some point it would be measurable. But I take your point and you are of course quite right no, the pressure would not noticably increase from a bowl of water left out. That sounds about right. The atmoshpere of Mars is relatively thin, atmospheric pressure on the surface varies from around 30 Pa (0.03 kPa) on Olympus Mons to over 1155 Pa (1.155 kPa) in the depths of Hellas Planitia, with a mean surface level pressure of 600 Pa (0.6 kPa). This is less than 1% of the surface pressure on Earth (101.3 kPa). -- Wiki -- lets see if I`ve got this right, you say 700-900 Pa, so if we be generous and make it 1000 Pa (or 1 KPa) and here on Earth 101KPa is standard room pressure, then it`s well over 1/100`th of Earths pressure on Mars? correct? Link to comment Share on other sites More sharing options...
insane_alien Posted July 8, 2007 Share Posted July 8, 2007 okay, for a thought experiment, we'll assume that mars is a uniform sphere at say, 0*C(i have the data for the vapour pressure of ater at this temperature handy). and that its atmosphere has a surface pressure of 1kPa(for the sake of arguement) and is bone dry(no water vapour). say you then coated mars in a layer of water. the water would evapourate till it had a partial pressure of 611Pa in the atmosphere. then it would reach equilibrium. so the final pressure of the atmosphere would be 1kPa + 611Pa which is 1611Pa. this is the maximum pressure you would be able to get on mars with water. temperature changes are neglected because these would eventually disappear as the planet regained thermal equilibrium. Link to comment Share on other sites More sharing options...
igosaur Posted July 8, 2007 Share Posted July 8, 2007 Would I be right in then thinking that any further water added to the surface would then exist as liquid and not add to the atmosphere? Link to comment Share on other sites More sharing options...
insane_alien Posted July 8, 2007 Share Posted July 8, 2007 yep, you would be right in thinking that. Link to comment Share on other sites More sharing options...
YT2095 Posted July 8, 2007 Share Posted July 8, 2007 no Wonder Arnie didn`t look too impressed in "Total Recall" he started off like this and ended up looking like this Link to comment Share on other sites More sharing options...
imp Posted July 9, 2007 Share Posted July 9, 2007 Presumably, this is why there are no lakes or rivers left on Mars. Since the atmosphere became stripped away by the Solar winds, the pressure dropped and hey presto. What didn't freeze into the surface simple evaporated away. After reading through this thread, I went back to this post. Kindly explain the Solar winds more thoroughly to me, please. imp Link to comment Share on other sites More sharing options...
igosaur Posted July 9, 2007 Share Posted July 9, 2007 One of the most scientifically accurate films I've seen no Wonder Arnie didn`t look too impressed in "Total Recall" he started off like this and ended up looking like this Link to comment Share on other sites More sharing options...
swansont Posted July 9, 2007 Share Posted July 9, 2007 After reading through this thread, I went back to this post. Kindly explain the Solar winds more thoroughly to me, please. imp http://en.wikipedia.org/wiki/Solar_wind Link to comment Share on other sites More sharing options...
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