This demo has been used to show that water is a polar molecule. Somewhere (can't remember where) I heard that the reason that the water stream is bent is NOT because it is polar. As I remember, no alternate explanation was given. Is the attraction simply between an excess electrons on the balloon and protons from the water? Has anyone ever tried the demo with nonpolar liquids? If so, what happened?

People have tried this experiment with nonpolar liquids, and it works pretty much the same as for water. Hal Harris, of the University of Missouri -- St. Louis, reported deflection of streams of carbon tetrachloride, carbon disulfide and benzene.

The cause of the deflection is the charged balloon inducing a surface charge on the drop or stream of bulk liquid, just as a charged balloon attracts an uncharged balloon -- even though the uncharged balloon is made of non-polar molecules! Especially under conditions of low humidity, which allow static charges to accumulate, this effect can be very strong indeed.

Dipolar effects are not likely to be very strong compared to surface-charge effects. Dipolar attractions fall off quite rapidly with distance, faster than the 1/rē associated with electrostatic fields.

For a discussion, see I.D. Brindle and R.H. Tomlinson, "Deflection of falling solvents by an electric field," Journal of Chemical Education 52, 382 (1975).