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A common highschool chemistry example for the polarity of water is the experiment where stream of water is deflected under the influence of a charge (electric field).

Now I have come across some condradicting claims, namely this Youtube video from Veritasium (from around 1:50) and more importantly a publication by Ziaei-Moayyed et. al..[1]

These sources suggest that this phenomenon is not based on the polarity of water but charged impurity ions in the water stream. They also claim that the electric field could also only re-orient the molecules, but not deflect them as the electric field also has an opposite effect on the other side of the dipole. This has been bothering me and it is hard to find concluding evidence either way. What are your thoughts?

References:

  1. Ziaei-Moayyed, M.; Goodman, E.; Williams, P. Electrical Deflection of Polar Liquid Streams: A Misunderstood Demonstration. J. Chem. Educ. 2000, 77 (11), 1520. DOI: 10.1021/ed077p1520.
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    $\begingroup$ At first I thought there was no way this could be hard to answer, but it seems that there is actually debate about this phenomenon to an extent. I would lean towards agreeing with that journal article because I don't think it would be published without being reasonably sound. I also just found another paper which reports distortion of the surface of a nonpolar liquid using a Van de Graaf generator, which seems quite surprising. Read about that here. $\endgroup$
    – jheindel
    Sep 6 '15 at 6:13
  • $\begingroup$ Very interesting. Seems to be caused by induced polarization of the nonpolar molecules. Vertical offsetting requires stronger attractive force to overcome gravity, cohesion and rejecting coulomb forces (?), thus the Van de Graaf generator was needed. So is the distance between dipole sides great enough to cause noticeable acceleration due to coulomb's law? $\endgroup$
    – A. La
    Sep 7 '15 at 14:46
  • $\begingroup$ Since impurities are charge neutral in sum (so much negative charge in ions as positive), how would electric field deflect them ? $\endgroup$
    – Greg
    Apr 6 '17 at 14:57
  • $\begingroup$ If electric field cannot deflect electric dipoles, magnetic field cannot deflect magnetic dipoles (same force laws, only letter is changed) - which is off course not true.. $\endgroup$
    – Greg
    Apr 6 '17 at 15:01
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An electrostatic field will attract both conducting and nonconducting materials even though those materials have no initial charge through electrostatic induction. That is why you can pick up lint or aluminum foil with a charged comb or balloon. See this demo with a neutral aluminum can.

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  • $\begingroup$ Good point. The video was about inducing charge to a conductive material (aluminum) but polarization works for nonconducting materials as well. Why then the hexane stream is not deflected in this video? Is it because the attractive force is simply too weak to notice? $\endgroup$
    – A. La
    Sep 7 '15 at 14:56
  • $\begingroup$ Exactly! Water and alcohols are polar and are, but given sufficient voltage, even non-polar hexane would exhibit some attraction due to electrostatic induction. The degree of polarity corresponds roughly to the dielectric constant: hexane=~2; isopropanol=~20; water=~80 (macro.lsu.edu/howto/solvents/Dielectric%20Constant%20.htm). BTW, thanks for listing that video! $\endgroup$ Sep 7 '15 at 18:44
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The polarity of water?

Rory Geoghegan

The deflection of a stream of water by an electric charge is often cited as a demonstration of the polarity of water molecules. The idea has even found its way into science text books.

What’s wrong with the explanation? Water molecules are polar; this is true. However it is also true that they electrically neutral and that they are very small. If a negatively charged rod is held 10 cm away from a water molecule then the attractive force on the hydrogen side and the repulsive force on the oxygen side are, for all practical purposes, equal and opposite.

Easily disproved Here are some simple demonstrations that clearly show that the ‘polarity argument’ is unsatisfactory, the first being the most obvious.

  1. No deflection occurs if the water becomes a stream of separate drops, unless the charged rod is sufficiently close to the point where the stream becomes discontinuous. (This refutes the polarity argument.)

  2. Get a 500 ml plastic drinks bottle. Using an awl or nail make a hole (about 1 mm in diameter) in the side of the bottle a few centimetres from the bottom. Place the bottle on a large insulated block (e.g. styrofoam container) near a sink. Fill the bottle with water and arrange it so that a stream of water flows into the sink. If a charged rod is brought near the stream it will not show a continuous deflection. (It may show a slight deflection initially but it will not last.)

  3. Clamp a salt cellar on a retort stand so that the salt can flow out. Bring a charged rod near the salt stream. It does not deflect. (The charge separation in NaCl is greater than that of water; so are the charges. NaCl is about five times more polar than water but it is not deflected.)

  4. (Don’t try this one.) A continuous stream of mercury would be deflected by a charged rod even though mercury atoms are not polar.

  5. A stream of iron filings is not deflected by a charged rod. These results cannot be explained by the polarity argument.

The right answer The right answer is perfectly simple. Continuous conductors may be deflected by an electric charge. Water is a conductor and so if, for example, a negatively charged rod is brought near an unbroken stream of water coming from a tap then a positive charge can be induced on the part of the stream near the charged rod. The force of attraction between the two will deflect the stream. This cannot happen if the stream is broken into separate drops.

Rory Geoghegan

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  • $\begingroup$ Ordinary water isn't that strong a conductor. So I doubt that this explanation (roughly the equivalent of the magnetohydrodynamic effect of a moving conductor in a magnetic field) works. I suspect that the effect is the same as would happen if non-conducting polystyrene beads were dropped through an electric field. We know they deflect but polarity has nothing to do with it. A key part, BTW, is that the field is not uniform. $\endgroup$
    – matt_black
    Apr 20 at 8:56
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G. W. Francis "Electrostatic Experiments" 1844 showed that charge separation in water can be created by electrostatic induction (experiment 153). William Thomson (Lord Kelvin) later demonstrated that charge separation in water occurs even in the usual electric fields at the earth's surface and developed a continuous electric field measuring device using that knowledge. He then developed the water-dropper electrostatic high-voltage generator using the same concept. Others have since shown experimentally that the amount of charge separation in water is proportional to the electric field strength. I believe that a stream of water is deflected because the electric field induces macroscopic charge separation in the water and not because water has polarisable molecules that have reoriented.

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