I’ve tried to search for an answer to this question but I couldn’t find one although I’m sure it must have been discussed…
Say I had a machine that consisted of a tank full of water and some sort of covering that could exert pressure downwards on the water, with absolutely no room on the edges for water to escape. If enough pressure was put onto the water’s surface, what would happen? Could it be compressed enough to turn into ice even though it was at room temperature?
Say the tank could withstand infinte force outwards from the water and the top portion of the machine could exert infinite force.
In a word, no. Here you see a phase diagram. Note that when in the liquid phase you can increase pressure to enter the solid (cross the green line). Water behaves differently though. The boundary between solid and liquid water is represented by the dotted green line. Note that there is no place in the liquid phase that increasing pressure while holding constant temperature will make the water transition into solid.
This has to do with the odd habit of water to expand, becoming less dense, when in freezes. Funky stuff, that water.
However, if you insulated it, and then attached your sealed chamber to a decent vacuum pump, you could evaporate enough water to cause what was left (~6/7 if you started at room temp, and if I calculated right) to freeze.
So pressure won’t cause the water to freeze, but in this sense,** lack** of pressure will.
I note that it is possible to go from the gaseous phase to the solid phase, though. Does this mean that it’s possible to compress steam into ice? Or better yet, can we compress steam such that its volume is less than an equivalent mass of water or ice? Can we put highly compressed steam into a gas cylinder? If so, why isn’t this done to efficiently transport large quantities of water? Rather than lugging around those huge 20-litre bottles for your water office water cooler, you could buy a gas cylinder of steam which weighs the same but holds much more than 20 litres of H2O.
I hesitate to answer this, because I’m not sure if you’re joking. I’ve been whooshed before. Compressing steam into a bottle to more efficiently transport water? To make the steam as dense as water, you’d have to turn it into water. Once you do that, it’s essentially incompressible. You’d be better of getting dehydrated water, cause all you’d have to do is add water, and voila.
Let’s take a specific case. Those compressed bottles of gas run about 2000 psig. For water to be in the vapor phase at 2000 psig, it has to be 640 deg F. Saturated steam at that temperature and pressure has a density of 5.7 pounds per cubic foot.
If you compress it more by increasing the pressure, it condenses into water. At 2100 psig it would be water at a specific gravity of 37.5 pounds per cubic foot.
If you let it cool, it condenses into water at the density of water would be at that temperature. At 630 deg F it’s about 39.7 pounds per cubic foot.
If you let it expand, it stays vapor, but becomes superheated and loses density, becoming less dense than that original steam density of 5.7 pounds per cubic foot.
Water at room temperature is around 62.4 pounds per cubic foot.
Hey, I just wanted to apologize about all the non-SI units. I can work in SI of course, but all my tables and spreadsheets work in English units because of the business I’m in, and I’m not going to convert without good reason. If you want degrees Kelvin, or kilograms, or Paschals, or watever scientists use for pressure, you’re on your own.
Does this mean that it’s possible to compress steam into ice?
Yes, but as you see in a detailed diagram you have to be at sub-zero (Celsius) temperatures. Also compressing a gas increases the temperature, so it has to be fairly cold “steam” in the first place.
Or better yet, can we compress steam such that its volume is less than an equivalent mass of water or ice?
No.
[QUOTE=naita] Does this mean that it’s possible to compress steam into ice?
Yes, but as you see in a detailed diagram you have to be at sub-zero (Celsius) temperatures. Also compressing a gas increases the temperature, so it has to be fairly cold “steam” in the first place.{snip}
[QUOTE]
I’m betting you couldn’t compress steam into ice. Ice has a lower density than water. You’d have to expand water into ice, lowering both the density and the pressure. You can go directly from the vapor phase to the solid phase, any who ever scraped the frost off their windshield knows that, but ice melts when you put pressure on it.
Only ice I and one form of ice XI have densities below one. The rest would sink in a pond.
This applies only to regular ice (Ih) at temperatures between 0°C and -22°C. Below -22°C, pressure converts hexagonal ice to ice II or III rather than to a liquid.
The relationship between water and ice does not have a bearing on the change from gas to solid without going through liquid. If your temperature is low enough, then compressing the gas enough will turn it into ice.
Compressing ice will only turn it into liquid in a certain temperature range. More likely, you are adding heat when you are adding pressure, and that is what is melting it.
I also have to mention that I neglected the other forms of ice. Thus, my initial answer was mistaken, and you can get ice (of a different type than in your fridge) by compressing water enough at any temperature. As Squink mentions, the types of ice you can obtain this way are more dense than water.
All right, you have some magical universe where PV doesn’t equal nRT, fine. If you could compress steam without heating it you could compress it into ice. Find me a set of conditions, temperature and pressure, which would allow you to compress vapor into ice, of any form. Well, except for ice-9, cause that shit’s just scary.