Monona,

 

While teaching at Western, I moved away from the solutions with Pt wire loop, because the loop needs to be cleaned between solutions in order to avoid cross contamination and un-interpretable results.  The wires are generally cleaned by swishing several times in an acid solution, typically 1M or 3M HCl.  However, this use seemed to make my students become cavalier using acids and even splash it and not do anything to clean it up.  Also, the acid solution becomes progressively more contaminated, and therefore cross contaminates the solutions with each other, giving meaningless results.  I didn't like making the students lose respect for acids, and I didn't like that the results were hard to see and interpret.  My students did 6 known solutions and then 3 unknown ones (three of the same salts) and they had a lot of trouble with the identifications with the wire loop in solutions method..

 

I saw the wood-stick cotton swab method in the Flinn Catalog (although more recently they espouse soaking wood splints in aqueous solutions of the salts.  The cotton swabs are wet with distilled water (they keep a beaker on the bench with tap water half filling it) by holding the tip over the beaker and using their distilled water wash bottle to wet it and the beaker to catch the excess water.  Then, they placed a microspoon of the solid for that test into one of the shallow wells of a spot plate, roll the wet swab tip in the solid, then put that tip in the hottest part of the flame.  The colors were bright and unequivocal, and the students never failed to identify their three unknowns.  When the stick catches on fire, they place the flaming swab into the beaker of water.

 

I liked this method better than the wood splint method, because the salt stays on the swab, and since the salts don't burn, having it just as visible on the swab provided an easy way to demonstrate that.  (The metals in the salt get excited, moving an electron to a higher level, from which it falls emitting light.)  Since the salt doesn't burn or disappear, the students can SEE that burning it isn't what causes the color.  The salts don=E2=80™t change on the swab, and the students can see they don't.

 

So even if the wire loop method with solutions of the salts is easy, the dampened cotton swab rolled in the salts gives better results.  And it doesn't require students using acid, in a way that familiarity leads to disrespect or contempt.  And that leads to the potential of future accidents.

 

Meg

 

Meg Osterby
megosterby**At_Symbol_Here**gmail.com
"It's better to be careful 100 times than to be killed once." Mark Twain