My professor commented that water has a higher dielectric constant than alcohol.

Okay, but on what basis?

He didn't elaborate; he asked us which one seemed "more electric" and most answered water. I guess from intuition.

But why exactly is water "more electric"? I think it has to do with the relatively high extent of auto-ionization in water, correct? Alcohols are generally less polar than water and thus cannot stabilize ions as effectively as water can. And I've even seem some computers submerged completely in oil. I'm guessing that only works because oil doesn't ionize much and thus isn't able to conduct a lot of electricity - or enough to short out any circuits.

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    $\begingroup$ It's basically a measure of polarity, not ionization. It doesn't have to do with the ability to conduct electricity but with the ability to permit an electric field. Try researching permittivity. $\endgroup$ – canadianer Aug 21 '14 at 5:18
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    $\begingroup$ Asking what substance seems more electric sounds odd to me ;) $\endgroup$ – canadianer Aug 21 '14 at 5:18

Before I start, please note that I am not a chemist so be sure to verify what I say...

The electric conductivity of water depends on what is dissolved in it. For example, salt water will be far more conductive than distilled water. So if you're professor was talking about sea water, the answer could actually be different from purified water.

Due to the dissolved salt ions, sea water is a better conductor of electricity and so by definition it will be a poor dielectric, since according to this Wikipedia article: to be a good dielectric a substance must be both a good insulator and able to be polarized by an electric field.

Purified water on the other hand will be a better dielectric because it does not contain any significant amount of ions and is therefor not very conductive (i.e. is a better insulator than sea water).

I think (but don't quote me on an exam) that the reason alcohol is a poor dielectric is because it is not nearly as polar as water is, and according to the above, the second requirement for a dielectric is that it could be polarized by an electric field.

(This might also have to do with the size of the molecules, since water molecules are generally smaller than many kinds of alcohols, and can therefore re-orient themselves more easily in the presence of an electric field, but this is a guess.)

The Solvent Classifications section of this Wikipedia page also mentions that the polarity of a solution can be roughly gauged by the solution's dielectric constant. So this too seems to support the idea that polarity and dielectric properties are related.

If anyone has any improvements or corrections, please let me know in the comments and I'll update the answer accordingly.

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    $\begingroup$ What do you mean that salt water is more conductive than distilled water but is less dielectric? $\endgroup$ – Dissenter Aug 21 '14 at 16:37
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    $\begingroup$ The salt ions in sea water make it a better conductor of electricity than purified water. The reason this makes salt water a poor dielectric is because by definition: a dielectric "is an electrical insulator that can be polarized by an applied electric field". Therefor, since sea water is a better conductor, it would be a poorer insulator and hence a poorer dielectric. I guess one could say that the conductivity and dielectricity of a substance are inversely related, with the added requirement that to be dielectric it must also be polar. $\endgroup$ – Vladimir Aug 21 '14 at 18:24
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    $\begingroup$ @Dissenter, I updated my answer with a few more details to make it a little clearer because upon rereading it, I found that I had some confusing wording in it. :) $\endgroup$ – Vladimir Aug 21 '14 at 18:51
  • $\begingroup$ The ability to conduct electricity and the dielectric constant are not particularly or directly related. The answer confuses the two concepts and doesn't really address the intent of the question. $\endgroup$ – matt_black Jan 18 '19 at 12:17

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