Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne 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


Click here to return to the Chemistry Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory