Monday, May 21, 2012

Swirling Round Bottoms: Viscoscity and IMFs

We swirled these liquids to see the effect of intermolecular forces on viscosity.
 This is a new demo that I worked up this year to help demonstrate the impact of intermolecular forces (IMFs) on the properties of liquids.  I found it in "A Demo a Day, Volume 2"; a great resource for quick and easy chemistry demos.  These beautiful liquids represent a range of IMFs, from the weakest (London dispersion forces) to the strongest (hydrogen bonding).  The purple liquid is hexane, with a little iodine for the beautiful color.  The orange liquid is ethanol, which has on O-H group that provides hydrogen bonding.  The blue liquid is water, a molecule that is famous for its efficient hydrogen bonding.  The bright green liquid is antifreeze, which contains mostly ethylene glycol, an organic compound with the same structure as ethanol but with an O-H group on each end.  The yellow liquid is glycerin, also an organic compound but with three O-H groups.
Glycerin vs. Hexane
 Swirling each flask is an easy way to see the difference in the viscosity of the liquids.  The stronger the IMFs, the less these liquids will swirl.  The hexane will swirl very easily and for a long time.  The swirling becomes increasingly sluggish as you progress to ethanol, water, ethylene glycol, and then glycerine.  Actually, the glycerine doesn't swirl at all.  It barely moves in the flask, much like cold honey.  I passed around the flasks to the class to get them to interact with the concept of IMFs.  It's hard to get the kids to understand the difference between the three IMFs and to appreciate how they affect the properties of a substance.  This easy demo sparked some good conversations about this difficult topic.
Everyone swirl!

Just kidding, they didn't drink their samples.
I just can't get enough of these cute little round bottom flasks.