how do they make dry ice? when did they first make it? how cold is it? when does it melt (evaporate)? can it be different temperatures?
::Pulling old Chemistry degree out and blowing off the dust::
Dry Ice is made by a multistep process. first, CO[sub]2[/sub] is isolated and cleaned, with all moisture removed (H[sub]2[/sub]O). It is compressed and shot onto an extrememly cold survace. The CO[sub]2[/sub] "freezes to form the dry ice. It is scraped off and compressed to form blocks for shipping.
If I recall correctly, it’s -77 C (that’s 196 K). It doesn’t melt, it sublimes. It goes from solid directly to gas, bypassing the liquid phase.
Yes, it can be different temperatures, provided all of those temperatures are below -77 C. Just like water ice. Water ice can be 0 C, or colder, but not warmer. (barring wacky chemistry and physics)
Back in the lab, we used to make a vapor trap on the high-vac equipment with dry ice in an alcohol bath.
Dry ice is quite cheap, actually. We used to sell it at Univ of Md, Chem. Dept. for $0.17/lb. In the “old” days, you could buy dry ice wherever you bought ice cream (parlors or soda fountains). Ask dad or grand dad.
The true question is, “If you fill a swimming pool with dry ice and let it melt, can you go swimming without getting wet?”
The answer is, “Yes, but don’t go off the high dive!”
In St. Louis, some ice cream places still sell dry ice. Bait shops might be another source.
You can also make your own with a cone and cloth bag device hooked to a cylinder of liquid CO2. It’s more like snow rather than the compressed cakes most of us are used to. I’m pretty sure it has to be liquid.
I had a CO2 tank I used to refill the cylinders for a target pistol. I’ve had it refilled at a place that services fire extiguishers.
Dry Ice can also be made without the cold surface. You just need a tank of compressed CO2, hook a small, heavy cloth bag (Ive seen it done with a tied up shirt sleeve) to the outlet, then turn the valve on. As the gas expands out of the tank it becomes cold (think evaporative cooling). Eventually dry ice will form in the bag. I’m sure this method is a lot more wasteful than directing a stream of CO2 onto a cold surface.
Dammit Gary! I’m too slow for this game.
now whats the deal about cold water ice. I thought you could only get ice so cold and then sublimates.
justinh, I get the sneaking suspicion that it’s lunch period and you’re on the library computer. Chemistry class is next and you’re not finished your homework.
Cold water ice? What, pray tell, is this?
no actually i am an OB. (old bastard)
I was told by some engineers that ice cant get below a certain temp or it goes to a gas (sublimates?). we were dicussing my idea about getting rich. a ice maker for a bar that would give super cold ice cubes that would keep your drink colder longer. but they said “nope, water would just turn to ice then to a gas as you lower the temp”
You can get ice as cold as you want to. Basically it forms at 32F, so you can’t really form colder ice. But once it forms, you can put it in a freezer and get it colder.
If the air is not saturated with water, then the ice will sublime [convert to gas. This is the same effect as water evaporating]. In a freezer it will be a slow process so you will still be able to keep very cold ice.
And don’t breathe either!
Once when I was in 2nd grade, I saw a block of dry ice evaporating in the sink. It looked real strange, as the CO2 evaporated, filled the sink, overflowed, and spilled to the floor without wetting anything.
Then, in a truly inspired flash of stupidity (remember, I was only 7 at the time) I wondered what CO2 smelled like. So I took a nice big breath of the stuff and nearly passed out on the floor.
what makes ice sublimate? tempature? humidy? air pressure?
It’s just evaporation. The higher energy on the outside of the ice surface can cause the water molecules to escape into the atmosphere, thus water vapor gas. It merely bypasses the liquid stage, since you don’t really have enough to form a liquid.
Would I be correct in saying that a single molecule of anything is neither a liquid, gas, nor solid?
Yes. The phase of something (i.e., whether it’s solid, liquid, or gas) is what’s called a “bulk property.” Meaning, you have to have some “bulk” of them to tell what the property is. While what designates “bulk” is actually sort of a frontier in certain research areas (micro vs macro vs meso), it’s safe to say that one molecule is neither solid, nor liquid, nor gas.
Why is phase a bulk property? Well, the phase of a chemical is dependent on its intermolecular forces, i.e., the forces between different molecules. If you only have one molecule, you can’t exactly have intermolecular forces!
HTH,
Quix
what makes ice sublimate? tempature? humidy? air pressure?
See:
http://invsee.asu.edu/ed/phase/phasefeat.htm?x=164&y=8
Notice the point labeled t.p. (triple point). At this temp & pressure you would have some solid/liq/vapour all at the same time. Where the t.p. is located depends upon the
substance.
For dry ice, the t.p. is at a pressure which is higher than atmospheric pressure. So at “room pressure”, as you increase the temp. (draw a straight horizontal line accross the diagram at a pressure lower than the t.p.) the dry ice goes directly from solid to vapour. If you put the dry ice into a container and subject it to high pressure, you’ll get a liquid.
With regular ice (water), the t.p. is at a pressure which is lower than “room pressure”. So as you increase the temp. (at a pressure higher than the t.p.) you go from solid to liquid to vapour. But if you were to put ice under a vacuum, you could get it to sublime.
So it depends on temp and pressure, but not humidity. (The evaporation rate of water depends on humidity, but that is not connected to this problem).