I read somewhere that CO2 can't be liquid in open air. Can some one explain why?
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Let's say you have some compressed CO2 you let it out of the gas tank wich causes it to freeze (CO2 snow) and then let it in the open air. Or you just take a block of dry ice. It will go directly to it's gas form and it will skip the liquid part (Sublimation).
But why o why doesn't it get liquid first?
It has to do with pressure. CO2 has no liquid phase at pressures below 5.1 amu. So it is possible to get liquid CO2, you would just need to have a lot of pressure. That's why you don't get it in open air. It's easier to get that kind of pressure in a closed container.
I think what I just said is correct. Hopefully one of the more scientifically knowledgeable people on this forum will read what I put and correct any mistakes if there are some and add onto what I said (I have a very limited amount of knowledge in this area).
You are correct: the triple point of CO2 is 5.11 atm and -56.4 deg C. Therefore CO2 only exists as a liquid at 5.11 atm and above.
Even if you froze CO2 and released it into the open air, the solid would sublime. Subliming is turning directly from a solid to a gas, without the intermediary liquid stage. Dry Ice is a perfect example ... isn't that CO2 anyway?
The reason it sublimes is that the solid is too cold to just melt. If there were enough pressure, or it were cold enough, there would be a liquid phase. Think of it like searing the outside of a piece of meat without cooking the middle all the way through. Bad example, I know, but it's the first thing that came to mind.
Look up the phase diagram of carbon dioxide (or any other substance for that matter). The theory behind this can be found in a physical chemistry textbook like Atkins or Ball, I don't feel like trying to write equations here.
Look at the phase diagram of CO2 and it should be liquid at a certain temperature and pressure. CO2 at room temperature is gas.
There may be a very slight but highly temporary liquid state. At normal Earth atmospheric pressures and temperatures, CO2 can only exist in the gas state so at these conditions, CO2 goes through the liquid state instantly.
The same is true for many gasses e.g. nitrogen, argon, CO and others.
| Dennise wrote: |
There may be a very slight but highly temporary liquid state. At normal Earth atmospheric pressures and temperatures, CO2 can only exist in the gas state so at these conditions, CO2 goes through the liquid state instantly.
The same is true for many gasses e.g. nitrogen, argon, CO and others. |
i'm not sure it's correct to talk about an very short-term liquid state during sublimation. What basically happens when a chunk of solid CO₂ is in STP is that it "explodes".
When a normal substance melts and then evaporates, the crystal lattice structure of the solid breaks down, but the molecules maintain their inter-molecular bonds (liquid form) until they absorb enough enough energy to break them completely (gas). What happens during sublimation is different. The molecules just fly off, like they were shot off of the lattice. There are no intermediate inter-molecular bonds.
Imagine a bunch of CO₂ molecules in a lattice - like in a block of dry ice - at 10 atm. These molecules are held together by the lattice bonds, and they are being squeezed together by the pressure of 10 atm. When you add heat, the molecules start dancing around enough to break the lattice bonds, but because they're still squeezed together by 10 atm they clump up... and you get liquid CO₂.
At 1 atm, but a very low temperature, the molecules are in the lattice as before. And again as you add heat, the molecules start dancing around enough to break the lattice bonds... but then because there's nothing else to keep them around they just fly away. Quite literally, the molecules just blast off.
So there is no temporary liquid state. You have a solid - with the molecules ordered in a lattice - then you have a gas - with the molecules flying off unrestrained. There is no liquid phase.
