## Why does dry ice sublimate?

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Markmilleru2
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### Why does dry ice sublimate?

Can someone please describe me Why does dry ice sublimate?

gmalivuk
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### Re: Why does dry ice sublimate?

This looks like it might be a homework question. Have you thought about it at all, or looked for the answer yourself?

Water ice can sublimate, too, it's just not the primary phase change we are used to seeing.
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Pfhorrest
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### Re: Why does dry ice sublimate?

I find this question curious myself, so I'm going to take an educated guess at an answer:

A better form of the question might be "why doesn't CO2 have a liquid phase?" Why does it go straight from solid (dry ice) to gaseous, sublimating?

A better form of that question might add the qualifier "at the temperature and pressure we're observing it at". In an ordinary room at standard temperature and pressure, CO2, as it warms from its frozen state to the temperature of the room, skips the liquid phase. Why?

Can it be liquid at some other temperature and pressure? My guess would be that yes, at much higher pressures, you can probably make liquid CO2. In fact I think you can buy tanks of highly-pressurized liquid CO2, like you can buy liquid nitrogen? Not sure about that.

As to why CO2 behaves differently in that regard than, say, water, my guess is that the bonds between CO2 molecules are much weaker than those between water molecules (water molecules being neat little magnets that love to stick together and all that), so it takes a lot more pressure to get them to hold together in a liquid form than it does water.

Though that then makes me wonder why isn't it commensurably harder to make it solidify too. Why, when cooled enough at standard pressure, are CO2 molecules happy to jump straight into solid form? Why don't they form a liquid first? This is basically back to the first question, but from the other direction.

My next thought is: are really happy to form a solid if cooled enough at standard temperature? Would a room cooled down to dry ice temperature have all the CO2 freeze out of the air? My thought is probably not, so maybe dry ice is manufactured under severe pressures -- pressures where maybe it does form a liquid first, and then forms a solid -- but when it's allowed to thaw at standard temperature and pressure, it just jumps straight to the gas that it'd rather be in those conditions.
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Tyndmyr
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### Re: Why does dry ice sublimate?

Pfhorrest wrote:My next thought is: are really happy to form a solid if cooled enough at standard temperature? Would a room cooled down to dry ice temperature have all the CO2 freeze out of the air? My thought is probably not, so maybe dry ice is manufactured under severe pressures -- pressures where maybe it does form a liquid first, and then forms a solid -- but when it's allowed to thaw at standard temperature and pressure, it just jumps straight to the gas that it'd rather be in those conditions.

Happens on mars, so yeah, probably. CO2 snow is a thing. Given the martian atmosphere, don't think high pressure is required.

It's liquid in fire extinguishers/compressed bottles for paintball guns, but if you spray one directly on something quite close for a bit, you'll likewise get frozen CO2 pretty easily. Accidentally did that a few times when playing paintball in the winter. So it seems that pressure is required to keep it liquid, not to make it solid.

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### Re: Why does dry ice sublimate?

We can look at the phase diagram of CO2 to see how it behaves, but of course that doesn't explain why it behaves that way.

Courtesy of Wikipedia (click the image for a SVG).

Pfhorrest
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### Re: Why does dry ice sublimate?

I feel like there's a more general insight to be had somewhere around here about why anything is ever a liquid (or not). If the bonds between molecules are strong enough, or the pressure is high enough, or the temperature is low enough, then they'll stick together into a solid. If they're weaker, or the pressure is lower, or the temperature is higher, they'll start to shift around a bit into a liquid... sometimes... and then eventually fly apart into a gas. What is the elaboration of that "sometimes"? What are the necessary and sufficient conditions for molecules to be bound just-so-slightly-loosely enough to form a liquid, and why do some but not all transitions from the conditions that create solids to the conditions that create gasses skip over those conditions?
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Zamfir
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### Re: Why does dry ice sublimate?

This picture is at least part of what you're looking for:

In general, you find a substance on the lowest value for the gibbs energy (H- TS). On this graph, the lowest value is on the blue solid curve for low T, on the green liquid curve for intermediate temperature, and on the red curve for high temperatures.The cross over points are phase transitions - at such a T, two phases have the same gibbs energy and both phases can therefore exist together. One phase with low enthalpy (H) but also low entropy (S), and a second phase with high enthalpy but also high entropy, but with equal H-TS for both.

Now, the graph above is only for 1 value of pressure. At other pressures, the curves look different. Very very roughly said:
-the gibbs energy for solids and liquids are only weakly dependent on the pressure, so those two curves stay similar over wide range of pressures.
- The enthalpy of a gas is also not too dependent on pressure.
- But the entropy of a gas is very dependent on pressure, with higher entropy for lower pressure
- At a lower pressure /higher entropy, the red graph for G=H-TS slopes down faster. And the cross-over point with the solid line shifts to the left
- Therefore, at low enough pressure, the crossover solid-gas comes before the crossover solid-liquid.

Pfhorrest
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### Re: Why does dry ice sublimate?

Thanks! That does help explain things somewhat. I'm not entirely clear what the Gibbs potential is (any relation to his free energy? Is that the same thing as enthalpy?) but even still this kinda makes some sense.
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chenille
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### Re: Why does dry ice sublimate?

Pfhorrest wrote:As to why CO2 behaves differently in that regard than, say, water, my guess is that the bonds between CO2 molecules are much weaker than those between water molecules (water molecules being neat little magnets that love to stick together and all that), so it takes a lot more pressure to get them to hold together in a liquid form than it does water.

Interestingly, it doesn't look like it. As people have noted, you only get liquid CO2 above its triple point, which is about 5.2 atmospheres. Besides pressurized tanks, by the way, it sometimes occurs naturally in the deep sea.

But though CO2 is still decently polar, that's actually a relatively high pressure. If you look for instance at wikipedia's table of triple points, you find CO2 right near the top. The highest they list is graphite, where the whole thing is held together by molecular bonds. But other things are all over the map – for instance ethylene is only 0.12 kPa, acetylene a good 120 kPa, despite both being similar hydrocarbons. It doesn't look like strength of intermolecular interactions tells you much of anything on its own.

To do better, you have to consider the structure of the solid as well. That's what Zamfir's diagram is about, but I think you can also do some rough explanations in more qualitative terms:

• For graphite the atoms don't just want to stick together, they want to do so at 120° angles, which makes the solid crystals very stable. It's going to take a lot of thermal energy to break that up, and at that point you need a lot of pressure to keep it from simply flying apart.
• Platinum atoms on the other hand hold together well, but with a metallic bond that doesn't much care about direction. The triple point has a high temperature but is very low in pressure.
• Noble gases don't really stick together at all, so the triple points are relatively low in temperature, but also high in pressure. I am guessing that's because once you have reached the point where the atoms are slow enough to stay together, it's also relatively easy to pack them into a regular arrangement, being basically spherical.
• Ethanol and butane on the other hand have weird shapes that probably don't pack well at all, which would make the difference between solid and liquid much less important. In one the molecules are sticky and in the other they aren't, but both end up with triple points at very low pressures.
CO2 is not as polar as H2O, and its triple point is at a lower temperature. But it looks like the molecules pack together really well, with each oxygen pointing right into a carbon. Whereas ice doesn't lose much by being disrupted – each molecule in water still makes something like 3½ out of 4 possible hydrogen bonds – I bet here it ends up a worse deal. So it may not take too much energy to break up the solid, but not before it is ready to fly apart into a gas, unless there is a lot of pressure to prevent that. That's my guess, anyway.

Zamfir
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### Re: Why does dry ice sublimate?

@ phorrest: Yes, the Gibbs potential is the same as Gibbs free energy. It's (enthalpy - temperature * entropy), or (H - T*S)

It's basically a particular form of the second law of thermodynamics.

When you melt a substance at constant temperature and pressure, you increase its enthalpy by dH. Which means you import heat from the rest of the universe at rate dH (per unit mass), and therefore you import entropy at rate (dH/T).

The second law says that total entropy must stay the same or increase. Therefore, the increase in entropy of your substance must be at least as large as the amount of entropy that you imported from the rest of the universe. dS >= dH/T, which becomes d(H -T*S)=<0 (if T is constant ).

(H -T*S) is a useful calculation term. You could say that it tracks entropy of the substance together with entropy in the rest of the universe. If (H -TS) goes down, total entropy of the universe has gone up, which can happen " by itself". So the phase with lowest (H - TS) is favoured.

Eebster the Great
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### Re: Why does dry ice sublimate?

chenille wrote:CO2 is still decently polar

CO2 is nonpolar. Remember that H2O is bent, due to the lone pairs on the oxygen atom, which is what allows it to be polar. Hydrogen bonding between water molecules gives the liquid phase of water many of its unusual properties. CO2 is linear, so there is no way for it to have a permanent dipole moment. Apart from that, carbon-oxygen bonds are relatively symmetrical anyway.

chenille
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### Re: Why does dry ice sublimate?

Eebster the Great wrote:CO2 is nonpolar.

You are of course right; I was thinking of the individual bonds. A C=O double bond does have some notable polarity to it, as seen in for instance acetone. Maybe not going to be much compared to water, but still a lot more than something like a hydrocarbon, which is what I was thinking about; it ends up with one of the higher pressure triple points but is not one of the least-sticky molecules. But that stickiness will have to depend on attraction between particular atoms, not on any over-all molecular moment.

Markmilleru2
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### Re: Why does dry ice sublimate?

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