M340: Molecular Structure – Polar/Nonpolar Liquids – Stream Deflection
Introduction
When a charged object such as a rubber balloon is brought close to a fine stream of a polar liquid, the stream is attracted toward the electrical charge. If the same charged object is brought close to a stream of a nonpolar liquid, the stream flow remains unaffected.
You can draw the structural formula for water and hexane or build models of these compounds to demonstrate why one is polar and the other is not.
To Conduct Demonstration
- Two burets are provided on a ring stand. One filled with a colored polar liquid (water), the other contains a colored nonpolar liquid (hexane). Large crystallizing dishes are placed under the burets to catch the liquids. Keep the liquids separate to facilitate recycling and/or disposal.
- Rub a plastic rod on a piece of wool cloth or your clothing. The rod easily obtains an adequate charge. Open the stopcock on the buret containing the nonpolar liquid and adjust the flow to a fine steady stream or rapid drops. Bring the chargedrod close to the stream; there should be no deflection of the flow. Repeat this procedure with the polar liquid; there should be a very noticeable stream deflection. Avoid getting the rod wet if you wish to recharge it.
- You can also use other charged objects such as a balloon or a round bottom Florence flask. However, the glass requires vigorous rubbing with wool to produce a sufficient charge. The polar stream will be attracted to both negative and positive charges because the polar molecules are free to rotate their oppositely charged ends toward the charged object.
- Provide a white board to provide a light backdrop to see the demo more clearly.
Safety
Wear safety goggles and avoid getting hexane on your hands. Do not use the hexane near an open flame. Keep the hexane separate from the water for recycling and/or disposal.
References
L.R. Summerlin and J.L. Ealy, Chemical Demonstrations: A Sourcebook for Teachers, 1987.