Sublimating solids

[Fiver]

Yesterday a friend was talking about dry ice for some reason, and I told him it sublimes: it goes from a solid to a gas without first becoming a liquid.

Why does it do this? I mean, I know why, in the sense that within the normal range of temperatures on Earth and at 1 bar of pressure, it skips the liquid state. But what properties of CO₂ cause it to behave this way?

Are there any places on Earth where liquid CO₂ can be found naturally?

What other common substances also sublime?

[Duck Duck Goose]

Chocolate. I know this is true because every time I look at the bag of Milky Ways out in the kitchen, there are fewer of them in there, but all my kids insist they haven't been eating them, so it must be sublimation.




Seriously, though--iodine.

http://www.xrefer.com/entry/516625

In chemistry, the evaporation of a solid without melting. For any substance the liquid phase only occurs within certain limits of temperature and pressure--if the pressure is low enough, heating a solid will result in sublimation. Substances that sublime at atmospheric pressure include carbon dioxide (dry ice) and iodine.

[stuyguy]

Mothballs.

[CalMeacham]

Lots of substances sublime, but I don't know how you predict which ones will. I know that Ferrocene does -- it's an organic molecule with Iron (Fe) right in the middle. I had to synthesize the stuff for an undergraduate organic chem lab, and it smells like soured milk. You can filter it all you want, but it still comes out a dirty off-white. If you purify it by sublimation, though, it forms pretty (if malodorous) orangle needle crystals. The substance evaporates when heated, then condenses on a surface you cool with ice. You can do it in a pair of petri dishes (bottom one on a hot plate, top one with a beaker of ice on it), but there are special sublimation chambers for bulk processing.

[micco]

I believe regular ice sublimates. Ice cubes left in open trays in a freezer slowly decrease in volume until you're left with a crust of formerly-dissolved minerals.

[Cap'n Crude]

Originally posted by micco
I believe regular ice sublimates. Ice cubes left in open trays in a freezer slowly decrease in volume until you're left with a crust of formerly-dissolved minerals.

AFAIK, that only happens in Frost-Free^TM freezers. A regular freezer accumulates ice and doesn't cause it to magically disappear.

And I was always taught that the verb was "sublimate." Yet everywhere I look it's just "sublime" now. Did it change, or were all my teachers stupid?

[Bob Scene]

Originally posted by Cap'n Crude
And I was always taught that the verb was "sublimate." Yet everywhere I look it's just "sublime" now. Did it change, or were all my teachers stupid?

I've heard at least one chemist say that you should always say "sublime" and never "sublimate," but they're both in the dictionary.

[Sunspace]

Regular ice sublimes outdoors as well, in areas where it gets cold enough with low humidity. _{Like Canada in February... whimper}

[Cliffy]

As a non-chemist, I always use "sublimate" as the alternative tempts linguistic confusion. Of course, of all the changes in the phases of matter, sublimation is by far the lovliest, so maybe it's not so bad.

--Cliffy

[NutMagnet]

I've always thought that sublimation was really the same solid-liquid-gas process that any element would go through under its required conditions (temperature/pressure/humidity etc.), only happening at a rate at which the time of the liquid state is drastically reduced.
For instance, ice in a frost free freezer actually melts (during the frost-free cycle) and evaporates. Similarly, isn't dry ice really melting and evaporating? Is this not so?

[Fiver]

Good point, NutMagnet. Is it really not skipping the liquid state, but just passing through it too quickly to count?

I know my OP referred to Earth conditions, but I'll note here that water ice sublimes on the Martian surface.

Again, is there any place on Earth where CO₂ exists as a liquid? Like, maybe deep under the polar caps?

[Mangetout]

I had in mind that you could work out which substances will sublimate and under what conditions by plotting state vs temperature vs pressure, I found a good example of this here

[Mangetout]

I had in mind that you could work out which substances will sublimate and under what conditions by plotting state vs temperature vs pressure, I found a good example of this here

Turns out that liquid CO² is possible, but not at normal atmospheric pressure.

[sford]

Originally posted by Fiver
Good point, NutMagnet. Is it really not skipping the liquid state, but just passing through it too quickly to count?

I believe not. Molocules are basically flying off the surface of the solid, without adhuring to their neighbors in a liquid fashion.

The same thing is true in reverse when the vapor deposits on the surface in the form of frost - it doesn't make little puddles of liquid that then freezes. The molocules of vapor connect to the surface, aligning themselves in whatever form the solid naturally forms.

(fyi - frost forms when the temperature of the surface is at or below the dew point ... see http://boards.straightdope.com/sdmb/...threadid=96629 for some additional discussion of dew points.)

[sford]

Oops - I meant to add that I saw two amazing things in a physics demo many years ago. The prof had a small sealed glass vial with liquid CO2 in it. It was under high pressure (about 800 PSI); the CO2 would have immediately boiled away if the vial were opened. (If you look at a phase diagram of CO2, the liquid phase includes room temperature, but only at high pressure. See http://www.co2clean.com/snowform.htm for more info.)

Then he put the glass vial against an overhead projector, which I think may have been turned on its side. You could see the meniscus (sp?). The bright light of the projector slowly started heating the liquid, and you could see faint shadowy shimmers from the evaporating CO2.

He then started explaining about the liquid/gas critical point on the phase diagram, and said that whereas most substances have critical points at very high temperatures, CO2 has a critical point quite near room temperature. As he spoke, the meniscus of the liquid CO2 went sort-of *poof* and disappeared. The temperature and pressure of the CO2 was such that there was no energy change required to change phase. The vial then contained supercritical CO2, which had properties indistingushable from the liquid and gas phases.

Well ... it was very cool for a geek like me.