Polar Liquid Displaced by a Charged Rod

A stream of water (polar) from a burette is displaced by a rod that has been charged with static electricity.  For comparison, a stream of cyclohexane (non-polar) is shown to be unaffected by the charged rod.

Equipment

  • 2 burettes to hold 50-100 mL of the liquids
  • ring stand or equivalent to hold the burettes
  • Two 50-100 mL beakers to collect the dripping solvents
  • (optional) a ball of organic material on a thread to first show that the rod is charged
  • (optional) glass rod and piece of silk

Reagents

  • water (with food coloring added for visualization)
  • cyclohexane (with coloring added)

Presentation

  1. The plastic rod is charged with static electricity by rubbing with a piece of fur.  Moving the rod near the ball of organic solid on a thread shows that it is charged.
  2. Position one of the collection beakers below the water burette on a ring stand and open the valve on the burette to get an even stream of liquid flowing.  This is readily visualized in a large classroom by projecting the stream with a document camera.
  3. Bring the charged rod near the stream and show that the liquid stream is displaced by the rod.
  4. Close the water burette and switch to the cyclohexane burette.  Get a stream of cyclohexane flowing into a different collection beaker.  Using two different collection beakers avoids having to separate the liquids when you are finished, and enables you to reload the burette during presentation if needed.
  5. Bring the rod near the cyclohexane stream and show that it is not displaced.

Alternative activities:

  • If you carefully touch the charged rod to the ball of organic solid on the thread, you can transfer a charge to it, and then, after rubbing the rod with fur again, show that the rod repels the ball rather than attracting it.  Then, rubbing the glass rod with the piece of silk can transfer the opposite charge to it; the glass rod attracts the charged ball on a thread while the plastic rod repels it.  Both rods, however, will attract the stream of water.

Details

A common explanation for the effect shown here is that the water is polar, and the molecular dipoles align to an electric field, thereby causing the partially positive end of the dipoles to be attracted to the negatively charged rod, or the partially negative end of the dipoles to be attracted to the positively charged rod.  This description makes the point for a Gen Chem audience that polar molecules align in electric fields whereas non-polar molecules do not.  It also makes the point (for the Gen Chem audience) that a charge and a dipole are attracted to each other, which is of fundamental importance for non-bonding intermolecular forces.  However, it has been shown fairly convincingly that this interpretation is flawed. [ref]

Safety

toxic

Special Safety Notes

  • cyclohexane is toxic and should be handled appropriately

References

  1. JCE article
water
Water stream is displaced by the charged rod.
cycl
Cyclohexane is not displaced by the rod.
setup
Most of the components used in the demo.