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Name: Brian 
Status: Educator
Age: 30s 
Location: N/A
Country: N/A
Date: N/A
Question:
I am an Earth Science teacher at the 7th Grade level. I have been looking for an experiment that will show the students how crystals grow in a short amount of time. I saw a professor once use mothballs made out of paradichlorobenzene once. He placed them in a beaker and melted them. Once they were all liquid, he placed them on a petri dish. They crystallized right in front of us. I have looked for moth balls that have that chemical and all I can find are the moth balls that contain Napthalene and it says that it can be fatal if inhaled. Any suggestions on this or another experiment.


Replies:
My favorite crystallization demonstration involves growing crystals of benzoic acid from water. Benzoic acid is fairly nontoxic stuff; it is used as a food preservative. I'm not sure if you can buy benzoic acid from anybody but a fine chemicals company, but it's a fairly cheap and available material.

To set up the solution, set some water boiling on a hot plate and dissolve as much benzoic acid in it as you can. (About 70 grams will dissolve in one liter of boiling water.) Then, pour the hot liquid water off any solid that remains. This is your starting solution. To conduct the demonstration, heat it on a hot plate until all the solid dissolves. Then take it off the heat; in a few minutes, beautiful needle-like crystals will form.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

Brian -

Paradichlorobenzene - PDB - is still available. You are right, Napthalene would not be a good choice. Just keep looking. I saw it last at an Ace hardware store. Be careful, PBD evaporates quickly after it melts and the vapors are very flammable. It is also very difficult to get it out of your glassware.

If you can't find it at the Ace, try a beekeeping supply place. Beekeepers use PDB to control wax moths. They can not use Napthalene.

Larry Krengel


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