Another “fact” that I need confirmation on from the most wise:
I foolishly got into an argument with a so-called “scientific expert” the other day. He claims that to make carbon dioxide (CO2) into a solid from a gas, you need to apply pressure to it as well as cool it.
This can’t possibly be true. If you put a gas into a closed vessel and reduce the temperature inside the vessel, eventually the gas will become a liquid, then a solid. That is, of course, unless the gas is like CO2, which we know proceeds immediately to a solid; or the gas is Helium (He) which as far as I know does not solidify at any temperature we’ve yet been able to reproduce.
I made the mistake of pointing out to my “expert” that water will freeze from a gas without pressure, to which he replied “I’m not going to argue with you, because you obviously don’t know what you’re talking about…”
OK, I admit that I wasn’t the best student in my department, but I did graduate from a good college with a degree in chemistry. Either I fell asleep far too often in class, or this guy is full of it.
So, please confirm this: CO2 will freeze if you lower the temperature, even if you don’t change the pressure. A cite from a good source would also be useful in case I run into his Nibs again!
You can find the phase diagram for CO[sub]2[/sub] on this page, about halfway down. As you can see, carbon dioxide will become a solid at one atmosphere if the temperature drops to about -78.5 degrees C. Liquid CO[sub]2[/sub] doesn’t exist below about 5.1 atmospheres. It seems to me your friend was thinking of the liquid phase, not the solid.
To make carbon dioxide (CO2) into a solid from a gas on an industrial scale, you need to apply pressure to it as well as cool it.
As pointed out in this cite the gas is pressurized to about 60 amospheres where it will liquify at room temperature and it is then released into an empty vessel:
I’ve done something like that, but on a much smaller scale, say a 1-pound block of dry ice. There was a tank of dry ice which connected to a snap-together box with a membrane on top. It basically looked like this.The highly-pressurized (we brought the CO2 pressure way down with a regulator, but we’re still talking pretty high PSI here) was forced through this membrane and thermodynamics took over, converting the gas into the solid. With some practice I got to where I could tell just by the change in sound when the block was finished without wasting CO2.
Perhaps he is arguing fromthe standpoint that you need to apply more than zero atmospheres of pressure in order for it to change phases. Strictly speaking, “applying pressure” is what’s done to just about everything on the planet by our atmosphere…
Cool! I had heard that CO2 in a fire extinguisher was liquid (and in your typical CO2 cylinder in a lab or restaurant). From the pase diagram, it appears that as long as it’s above 1100psi (normal for a gas cylinder), it will be liquid… up to 88F when it passes the critical point. So on a hot sweaty day, there’s super critical fluid in the fire extinguisher.
It’s a fairly common chemistry demonstration to put a piece of solid CO[sub]2[/sub] in two-liter plastic soda bottle and seal it. As the pressure builds up you can keep it liquid for a little while[sup]*[/sup]. I think most bottles can withstand something like 10 atm, but don’t go doing this unless you know what you’re doing.
[sup]*[/sup]… and then the bottle explodes. Or you release the pressure, almost as much fun. Note : if you throw the bottle in a pond to let it explode, be sure there are no ducks around. Not that the author would know anything about that. Just a precaution.
At sea level, atmospheric pressure (14.7 psia or 29.92 inches of Hg or 33 feet of water) is exerted on everything. By convention, atmospheres are expressed as an absolute pressure with 0 atmospheres being a perfect vacuum. Pressure readings in PSI are expressed as psia (pounds per square inch absolute) where 14.7 psia equals one atmosphere. You can also use psig which is pounds per square inch gauge in which 0 psig equals one atmosphere. The two conventions typically used for PSI may be the confusion that you allude to in your post.
