Ask A Scientist© Chemistry Archive

name         Michael
status       student
age          19

Question -   I do a lot of work with dry ice.
1.  Since it is made of CO2, is there any danger in breathing the air near
dry ice?  Is dry ice purely made of CO2?  If a room has poor circulation,
could the concentration of gaseous CO2 be significantly raised in a 600
cubic meter lab by sublimating a liter of solid dry ice?  How high would
CO2 have to be elevated to make a dangerous environment?  What is the
normal partial pressure of CO2 in "normal air" in comparison to that
dangerous level?

2.  In a post around #1000 to #1099, as a part of the QUESTION, someone
said that the visible, white vapor trails coming from dry ice are actually
water vapor.  Is that true, or is it actually CO2.  I notice that white
dry ice vapor tends to flow downward rather than radiate all around.  I
would expect water vapor to rise more than to fall, and I would also
expect CO2 to fall more than rise, based on their average molecular
weights in comparison to the average molecular weight of the average gas
molecule (0.8 times N2 + 0.2 times O2).  Does that line of reasoning sound
valid?

3.  But then if CO2 gas really does fall as dramatically as the white
vapor of dry ice, then do all present heavy gas molecules behave the same
(fall downward)?  How would CO2 gas be distributed from floor to ceiling
indoors at room temperature?  I wouldn't believe that they would all be
near the floor, where the white vapor eventually ends up... so maybe the
white vapor isn't completely composed of gas?  Maybe it contains some
liquid water rather than gaseos water or gaseous CO2?  I would expect gas
molecules to distribute by density to some extent (like how there is less
O2 at the elevation planes fly at... like phases of a liquid), but is the
falling of dry ice white vapor an accurate, macroscopic, observable
reflection of that distribution?  In a room only 3 meters high%2
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This is several questions:
1. Dry ice is solid CO2. However, if left exposed to air or liquid water,
water will freeze on its surface. If fact if dry ice is dropped into a glass
of water it becomes encased in water.

2. Dry ice should not be handled in confined spaces, but is not likely to be
a toxicity problem in an open room. CO2 gas at high concentrations is acrid
due to the formation of carbonic acid in the nasal passage; however, air
containing over 10% by volume CO2 is toxic. This was one of the dangers
faced by the astronauts in the aborted Apollo 13 mission to the moon.
3. The normal concentration of CO2 in air of course varies but is of the
order of only tenths of a percent or less.
4. The visible white vapor trails coming from dry ice is water vapor
condensed by the low temperature of the CO2 vapor. The vapor trail flows
downward primarily because the vapor is cold compared to room temperature
d1/d2 = T2/T1 where d1 and d2 are the molar density of CO2 at temperatures
T1 and T2 (expressed in absolute kelvins). The difference in molecular
weight plays a smaller roll. It is the temperature that is the major cause.
5. Gases significantly more dense than air also tend to flow downward and
"hug" the floor.
The classic example of this type of effect is the vaporization of gasoline
which can flow along the floor/ground to a source of ignition and then
"flash back" violently.

Vince Calder
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1.)
Of course I would have to say always be careful when dealing with any type of
material that can cause the displacement of breathable air.  The only danger
that is present is the carbon dioxide's ability to displace breathable air.  I
am fairly certain that there is no chemical rxn between oxygen and CO2 
that you
would need to worry about.  However, carbon monoxide is a different story.
"Significantly" raise the concentration...I do not know the numbers w/o
calculating them but in my opinion any build up of non natural gases that you
wouldnt breathe in a park w/ many trees I would consider to be significant and
would consider getting a fan and opening a door and window.

Here is a source on CO2 levels:
http://www.soest.hawaii.edu/oceanography/courses_html/OCN201/Chrisnotes/Glo 
balchange.html

2.)
Yes it is true.  The sublimation point of CO2 is around -79 °C.  The CO2 block
has no problem taking ANY moisture in the surrounding air and slowing the 
water
molecules down to the point of dew or even freezing.  The falling of the vapor
has more to do with its higher density due to the depressed temperature more
than anything.

3.)
Yes, just about any gas that has a specific volume that is very sensitive to
temperature will fall to the ground temporarily (until heated back up to
ambient temperature) if it comes in close proximity to the heat sink (CO2
block).
At room temperature and less than 5 atmospeheres of pressure the ideal gas law
will accurately apply.:  PV = nRT.........n / V can then be calculated = P /
(R*T).  However, I dont think that your numbers will be all that accurate 
since
the you're not considering the diffusion of molecules from one point to the
next.  Diffusion will happen just as a result of there being a concentration
gradient (or difference) from one point to the next.  There is no YES or NO
answer to this.  It is more like a probability problem.  You are right.  For
the most part, all things being the same, molecules will settle by density.
This eplains why our atmospheric pressure as a function of depth (or 
altitude )
follows an exponential function ( compressible gases ).  Using the same logic,
this is why scuba divers can calculate their depth based on pressure since
water is incompressible the depth is linearly interpolated by the pressure on
their transducers...Sorry you lost me on that last one.

-Darin Wagner
========================================================
1.  I suggest you get hold of the Material Data Safety Sheet for dry ice to
answer your questions about safety.  Ventilation will be important.

2 & 3.  As you surmise, the white is due to light scattering from water vapor
or mist (i.e., minute droplets of liquid water condensed from the air due to
the extreme cold near the dry ice) -- similar to the contrails behind a jet
plane.  Because it is condensed, it is much denser than air and flows 
downward.

     The relative densities of the gases will have a minor effect on the
vertical distribution of gases (in a room) as long as there is any convection.
A very heavy gas, such as sulphur hexafluoride, will accumulate near the floor
in a still room.  There is not enough difference in the masses of carbon
dioxide, water and air for this to be significant in a room with any kind of
motion.

     In the atmosphere, the distribution of gases at high altitude is
significantly different than at low altitudes.  All gases are less dense at
altitude but the relative amounts of oxygen and nitrogen also change with a
smaller percentage of the air being the heavier oxygen at high altitude.

Greg Bradburn
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