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Gases Trapped in Filter
Name: Jay
Status: student
Grade: 9-12
Country: USA
Date: Spring 2012
Question:
Would the toxic gases that are trapped in a charcoal filter gas mask be an example of a mixture, solution, or something else?
Replies:
It certainly would be a mixture. Generally, the gases are considered to be “adsorbed:” the molecules cling to the outsides of the charcoal particles.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
That is an example of adsorbtion, (with a "D")
which is a fairly common mode for mixtures of solid and gas.
Adsorbtion can have bond strengths ranging from
weak like of physical condensation (solids and liquids vs gasses)
to almost as strong as a chemical bond within a molecule.
It depends on the particular pair of adsorbing and adsorbed species.
Because the solid's structure remains unchanged,
the gas doesn't fully penetrate the solid, it just sticks on the surface.
The mixture remains pretty heterogeneous on the microscopic scale:
solid here, gas there.
That is one reason why it is not considered a solution,
even though the bonding energy there can be greater than that of most solutions.
To have great capacity to adsorb large gasses, the surface area must be great,
which is done by having very small particle-size or many small pores in larger particles.
To have great force in adsorbing the first bit of gas, the pores must be small,
with diameter not much larger than the molecule being adsorbed,
so that a given gas molecule gets to touch solid surface on more than one side.
Some adsorbers have both strength and capacity,
some have neither, many have one more than the other.
Activated charcoal is definitely very strong,
at least for organic vapors when it is empty of gasses,
and it has pretty good capacity.
For adsorbing water vapor, silica gel is strong but has low capacity,
and Bentonite clay is weak but has high capacity.
Many plastics absorb/desorb a few percent by weight of water vapor,
when cycling slowly between 1% relative humidity and 90%.
Notice I used "absorb" with a "B" this time, not "adsorb" with a "D".
For these plastics, gas wiggles into the amorphous matrix
of the plastic molecules, and swims slowly among them.
This is an example of a true solution of gas in a solid.
Jim Swenson
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Update: June 2012
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