Low-pressure Water
Most recent answer: 12/28/2011
- Randall (age 55)
Irving, TX USA
There won't be any significant fraction of the material in the form of little droplets. It will essentially all be in the main liquid pool or in the isolated molecules. The reason is that if a few molecules join up they lose almost as much entropy as if they joined the pool, but don't lose nearly as much energy.
There will also be almost no isolated H, H2, O or O2 atoms or molecules. They have much higher energy than the H2O form. Only under really extreme low-pressure vacuum conditions would you start to get an appreciable fraction of isolated atoms. The vapor pressure of liquid water is far higher than that.
Mike W.
(published on 12/28/2011)
Follow-Up #1: vapor pressure
- Randall (age 50)
Irving, TX. USA
B) I'm not quite sure what you mean by "an airless environment". If you just mean no air, but still water vapor, then the vapor pressure is as described in (A). The presence of air has very little effect on the water equilibrium vapor pressure. If you mean no gas of any sort, then you'd have to keep pumping to get close to that, and gradually you'd pump all the water away.
Mike W.
(published on 01/01/2012)
Follow-Up #2: partial vacuum above water
- Randall (age 50)
Irving, TX. USA
Let's say that you would like to keep the pressure low but really need to avoid gradually pumping the water out. Then the best you can do is to lower the equilibrium vapor pressure of water as much as possible. Here are the obvious steps to take, although I don't know which ones are practical in your application:
1. Lower the temperature of the liquid as much as possible, say to right near the freezing point.
2. Add non-volatile solutes (salt, sugar, antifreeze, ...) to the liquid. That both lowers the vapor pressure directly and allows you to lower the temperature further without freezing. That comes closest to the sort of step you were considering, but it only lowers the vapor pressure, not reducing it to zero.
Now if you need lower pressure than you can reach with these methods, you'll have to leave the pump on and live with the gradual water loss. In that case you might want to take some step not only to adjust the equilibrium vapor pressure but also to reduce the rate at which water leaves and enters the liquid. A layer of non-volatile oil on the surface would help to do that. There will be a basic limit making it hard to reduce the pressure much below the equilibrium vapor pressure, however, since at lower pressures the liquid will boil, releasing vapor regardless of what's on the surface.
Mike W.
(published on 01/02/2012)
Follow-Up #3: freezing by boiling
- Randall (age 50)
Irving, TX. USA
Water can boil at low temperature if the pressure is low enough. The boiling process actually soaks up heat and cools the water further. You can freeze water just by pumping on it with a vacuum pump. So if you pump a little too strenuously and start to boil the water, it won't heat things up and lead to rapid boil-off. Instead it will start to freeze the water and thus, if you're lucky, be self-limiting.
Mike W.
(published on 01/02/2012)
Follow-Up #4: water triple point
- Randall (age 50)
Irving, TX. USA
A) Yes, if you're going to use pure water the thing to do is to pump for a while until essentially all the air is gone, then leave the temperature just barely above the triple point. You'll have liquid in equilibrium with a minimal amount of vapor.
B) For calculating the vapor pressure-temperature relation, the Clausius-Clapeyron equation is what you're looking for. (See for an account, with some slight corrections.)
C) Especially if you use well-filtered water, you can supercool a few degrees for a pretty long time, reducing the vapor pressure further. I think you'll just have to try experimentally to see how far that can work.
Again, I'm not sure what your application is, but if you can use any sort of non-volatile solute in the water, that directly reduces the vapor pressure and lets you cool further without freezing, further lowering the vapor pressure.
Mike W.
(published on 01/03/2012)
Follow-Up #5: pressurized container
- Simon (age 52)
Uk
The sealed container might explode if it wasn't strong enough to withstand the pressure difference between the high-pressure inside water and the low-pressure outside water. So long as the container didn't explode, the water inside would be held too compressed to boil. It would stay liquid.
Mike W.
(published on 10/03/2015)
Follow-Up #6: pumping water through a closed cycle
- Forrest (age 37)
Georgia, USA
You can pump water through a closed cycle with virtually no leaks. A sealed liquid pump can be used at one or more points of the cycle. That's a different problem from Randall's, because he wanted to keep the vapor pressure low right above the water.
Mike W.
(published on 10/31/2015)