Tuesday, September 19, 2006

Maybe I'll get your attention with a list post

In honor of classes and labs starting up again for fall, and my colleagues in lab running off in the afternoons for their work as TA's (teaching assistants), I present:
"The All Time Top 3 Questions Students Asked Me in the Lab" blog entry.
Because of my funding situation, I've actually never TA'd a lab at Princeton (yes, we use "TA" as a verb). At Princeton, I lectured in weekly recitation/ problem solving sessions called precepts. I TA'd labs as an undergraduate; at my college many of the upperclass chemistry majors helped out around the general chemistry and organic chemistry labs to make a couple extra bucks, because there were no grad students.
Let me preface the list by saying that I thoroughly enjoyed being a TA. In fact, before stumbling onto this science writing gig, I wanted nothing more than to be a professor at a small college, milling about the labs every day. Even the most dedicated TA has to admit that redundant questions can be trying at times. I hope my post will help any gen chem/ orgo students that happen upon it (and maybe give all the current and former lab TA's out there a chuckle.)
If anyone has anything to add to my useful advice, please feel free to do so.
So, without further ado, the list of questions:
3. "Is this a precipitate?"
A precipitate is a product of a chemical reaction that is not soluble in the medium you've used to run the reaction. Precipitates come up in experiments that teach students important chemical concepts, like chemical equilibrium and general solubility properties of the elements. It can be tough to tell whether you've formed a precipitate when you've combined two solutions. It's important to make a note of the appearance of each solution before you combine them, and then compare to what you see afterward. I've seen precipitates range from cloudy white suspensions to yellow powder. Most of the time, the precipitate is more dense than the reaction solvent and sinks to the bottom of the test tube or beaker, but that's not always the case. This website has a couple of images of precipitates.
2. "Is this dry?"
In the organic chemistry lab, drying the solvent is one of the last steps in "working up" the product of the reaction, that is, isolating the product from the reaction medium and reagent byproducts. Many "workups" involve treating the reaction mixture with a solution of a base or an acid in water, and even if your organic solvent doesn't mix with water, there are probably small droplets of water that end up mingling with your solvent. You may be able to notice this if the solution is cloudy, or you see the droplets in there. To get rid of that residual water, we use "drying agents", a salt that absorbs the water. The drying agents I use these days are sodium sulfate and magnesium sulfate. At college, the labs stocked calcium chloride pellets. Regardless of what drying agent you use, the principle is the same.
-Make a note of your solution's appearance before adding drying agent. Is it cloudy?
-Add the drying agent a little at a time to the solution.
-Swirl the solution around for a little while (sodium sulfate is a little slower, so take your time there.)
-Watch for changes in cloudiness. If the solution's still cloudy, add a little more drying agent, etc.
-The best way someone's described how to tell when you're done is to look for "the snowglobe effect". Essentially, if the drying agent absorbs water, it clumps together at the bottom of the flask. If you add a little excess drying agent and the solution's dry, the drying agent swirls around like a snowglobe.
1. "Is this boiling?"
If you're asked this one, resist the urge to wonder whether your student has ever cooked pasta before. The evidence for boiling is usually the appearance of bubbles containing vapor from the liquid that rise to the liquid surface. Sometimes little bubbles form on the very hot bottom of the beaker before the liquid actually starts boiling, but don't be fooled.
See this wikipedia entry for a more detailed entry on boiling.

Finally, I think that the Not Voodoo website is a great resource for lab technique, and Zubrick's "Organic Chem Lab Survival Manual" is pretty good, too. If you're a little more experienced, I like "Advanced Practical Organic Chemistry" by Leonard, Gygo, and Procter.

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5 Comments:

At 2:34 AM, Anonymous Anonymous said...

How about: "which one is the organic layer?" It can get tricky, though, with those pesky more-dense-than-water solvents!

P.S. I like your blog.

 
At 1:45 PM, Blogger Carmen Drahl said...

That's a great one, too, thanks! Adding a couple of drops of water and watching to see where they go should do the trick. I love the more-dense-than-water solvents if I need to extract the product out of the aqueous (water) layer.

 
At 9:20 PM, Anonymous Anonymous said...

How about:

"What color is this?"

"Has it been in the ice bath long enough?" or "How long do I leave it in the ice bath?"

"How much mag sulfate do I add? " (a variation on the the "is it dry" comment).

And of course the generic "does this look ok?" or "do I do "x" next?" (as in "I'm too scared to believe what I read in the handout").

Sorry, I've been a TA/professor at a small college for too long. :-)

I'm sure there are more..

ps I like your blog too. I'll miss Dylan's as well.

 
At 12:10 PM, Anonymous Jordan said...

Nice blog, and I'm glad I just discovered it.

I second the recommendation for the Advanced Practical Org. Chem. book. I read it near the end of my PhD (which involved a lot of synthesis) and learned a lot of tricks that I had never seen before.

 
At 2:20 AM, Anonymous Anonymous said...

As far as dense comments go, the best one has to be "Where do I get hot water from?"
That had us in stitches, as the poor student stood wondering what was so funny.

 

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