I read somewhere that CO2 can't be liquid in open air. Can some one explain why?
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.
Errr.....I was told that the boiling point and the melting point of carbon dioxide are just too close and it doesn't need to become liquid first.
It's impossible or really hard to get liquid CO2 at 1 atm. However, you can make it liquid at about 56 atm at room temp.
I really wanna see that, but I guess I can't feel it.
I remember my first encounter with liquid CO2. I was playing paintball a few years back when the topic came up. I remember telling everyone that CO2 had no liquid state. I had learned in high school that CO2 went directly from a solid to gas (I have an awesome chemistry teacher who let us play dry ice.) One of the other guys that I was playing with told me that I was wrong and that CO2 really did have a liquid state. I argued with him for a long time until he handed me a 20 oz CO2 tank. As I held the tank, I could feel a liquid sloshing around inside of it. I was perplexed. Later, I discovered that CO2 does have a liquid state, but only under very high pressure (like in a tank.) I also learned about CO2's triple point. At the right temperature and pressure CO2 can be a liquid, gas and solid.
Mm, phase diagrams can be a bit of a mind-bender when you think in terms of solid→liquid→gas, but you ain't seen nothing yet. ^_^; There's still plasma and B-E condensates even beyond all of that, and then there are the really exotic phases like superliquids... but then you're getting into some real wacky physics.