Friday, May 4, 2012

Something Unexpected from Heptane

Setting up the burets.
Today I pulled out one of my "go to" demos to introduce the concept of polar and non-polar molecules.  Polar molecules, like water, will be attracted to an electric charge while non-polar molecules, I used heptane today, is not affected by a charged object.  Or so I thought!

I prepared two burets for the demo, one with water (with a little blue food coloring to make it easy to see) and the other with heptane.  I borrowed a plastic stick and piece of rabbit fur from the physics closet to generate a negatively charged object. 

When I passed the negatively charged stick by the stream of water, it bent toward the stick.  The challenge was to get the water to bend enough to hit another beaker.

Notice the cat fur and plastic stick, and the water stream bending.

A student hits the second beaker!
Heptane does not respond to the charged stick.

Here the heptane stream is not bending, just as I expected.
The stream of heptane was not as responsive to the negatively charged stick, at first.  But when I tried it another time, the stream of heptane bent toward the charged stick.  This observation was completely unexpected.  I actually exclaimed, "That's not supposed to happen!"  I tried it again, and again the heptane stream was attracted to the stick.  What we found is that if I passed the stick near the tip of the buret, the heptane stream would bend.  But, if I approached the stream farther down, away from the opening of the buret, there was no response to the stick.
What?!  This is not supposed to happen!  The heptane bent toward the stick.

Another look at the heptane doing what it's not supposed to.
 I have a few ideas about why this happened, but it requires further experimentation to develop a working hypothesis.  I tried a second batch of fresh heptane, same result.  So I took the heptane and tried to dissolve some food coloring into it.  In this case, the heptane behaved as any non-polar substance; the food coloring did not dissolve.  I'm wondering if anyone has seen this in their lab?  I'm going to rework this demo again this week to see if I can figure it out.
Here's some food coloring in the heptane, it doesn't dissolve
 Chemists often use the saying "Like dissolves like."  In the last part of the polar/non-polar demo I showed my students how this plays out.  I poured about 40 mL of water and 5 mL of heptane into a eudiometer tube.  Then I put a small pellet of iodine, a non-polar solid, in the tube.  As I slowly inverted the tube several times, the heptane layer took on a beautiful pink color from the dissolved iodine, but the water layer stayed clear.  The non-polar iodine only dissolved in the non-polar solvent.
The iodine only dissolves in the heptane, turning it pink.
 To make it even more interesting, I dropped some food coloring into the tube.  Food coloring is a water-based solution, so it should not dissolve in the non-polar heptane layer.  Saying that to the kids is one thing, but seeing it is really believing in this case.  The food coloring stayed in a tight drop and left no trace of interaction with the heptane, and then it began to spread out into the water layer.  It was very cool to watch.

Watching the heptane "bubble" float back to the top.

The final look at the polar and non-polar solvents.
Understanding the impact of polarity on the properties of a substance is one thing, but getting my students to identify which molecules are polar and which molecules are non-polar based on the structure is not as easy.


  1. SO, when does the experimenting being to solve the heptane mystery??

  2. I did the demo again on a rainy day. I could barely get enough charge built up on the plastic stick to bend the water stream. There was no response from the heptain.