Ask A Scientist Chemistry Archive

name         Tyler L.
status       student
age          15

Question -   Is it possible to create liquid carbon dioxide by blowing
into a balloon and placing the end on a beaker and using the extreme
temperatures of liquid nitrogen to cause the gaseous co2 into lco2? I
discovered this question while trying to find an inquiry project.
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No carbon dioxide liquid has a vapor pressure that is much greater than 1
atm. The experiment you describe will result in solid CO2 (dry ice). The
vapor pressure of liquid CO2 from 0C to 30C is:
Temp.(C)        V.P.(atm)
0                        34.4
5                        39.2
10                       44.4
15                    50.2
20                    56.5
25                    63.5
30                    71.2
So you would need a container that would withstand about 50 to 70 atm to
produce CO2. That is possible and is done routinely. However, it is not an
experiment for amateurs to try.

Vince Calder
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Yes you would pull out carbon dioxide, except you would just as likely condense
a lot of water, and any other compound in vapor with a higher boiling point
than liquid nitrogen.

Don Yee
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Tyler-
    Logical thing to try, but it does not work with CO2.
CO2 is a little different that way,
At the temperature where solid frozen CO2 (also dry ice) melts into liquid CO2,
    its vapor pressure happens to be higher than one atmosphere.
So, whenever the pressure is 1 atmosphere or less, CO2 just skips the liquid phase.
The solid evaporates into gas, or the gas "frosts" making dry ice.  No melting, no liquid, happens in between.
Which is of course why it is commonly called "dry ice".

The pressure in your balloon is only 1-10% higher than 1 atmosphere.
Not really different.
Instead you would need a metal pressure tank holding 10 atmospheres of CO2 gas pressure.
(Please do not try this without friends and knowledgeable supervision.)
Then the tank over your LN2 bucket would collect CO2 liquid at around -50C.
The best you could do is 3885mm pressure (~5 atm.) at -56.6 degrees C, which is the "triple-point" of CO2.
Colder than that, your liquid CO2 would all freeze solid.
Warmer than that, you would need higher pressure of CO2 than 5 atmospheres.

Water's triple-point has a vapor pressure well below 1 atmosphere,
so we on Earth commonly get to experience liquid H2O.
Outdoors on Mars, they would not.   Their ice-slush drinks would try to turn into clear dry sand.
The "sand" would never melt in the cup, just slowly vanish from top to bottom.
If you kept the lid on, then you'd actually get to see and shake some liquid wet slush inside the cup.
And the vapor would lightly breathe out through the hole until it was all gone.
It would be ten times the novelty it is here on Earth.

Jim Swenson
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