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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.
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Notice the cat fur and plastic stick, and the water stream bending. |
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A student hits the second beaker! |
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Heptane does not respond to the charged stick. |
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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.
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What?! This is not supposed to happen! The heptane bent toward the stick. |
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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.
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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.
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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.
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Watching the heptane "bubble" float back to the top. |
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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.
SO, when does the experimenting being to solve the heptane mystery??
ReplyDeleteI 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.
ReplyDelete