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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
    Commented Aug 21, 2014 at 5:18
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    $\begingroup$ Asking what substance seems more electric sounds odd to me ;) $\endgroup$
    – canadianer
    Commented Aug 21, 2014 at 5:18

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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
    Commented Aug 21, 2014 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
    Commented Aug 21, 2014 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
    Commented Aug 21, 2014 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
    Commented Jan 18, 2019 at 12:17
  • $\begingroup$ Thanks @matt_black! Could you provide some more details and clarification so I can point readers to your response? $\endgroup$
    – Vladimir
    Commented Apr 6, 2021 at 5:44
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The dielectric's is effectively a measure of the solvents ability to hold a charge and the Ɛr is a measure of its permittivity.

From a chemical perspective the degree of this is inversely proportional to the born energy of an ion (or self ionization energy) so a solvent that is better at holding an electric charge like water will lower the energy requirements for a species to become an ion. Look up the Born equation, there are a few key relations in there but the main one would be the E=1/Ɛr relation, bigger dielectric, lower energy requirements.

Polarity and dielectric's are not the same thing, polarity relates to separation of charges over space, dielectric's relate to permeability and strengths of electric fields, but yes they do share some strong similarities and trends and a lot of the time you can make assumptions like "this solvent has a higher dielectric constant, so therefore will be more conductive. When talking about salts, 'dissolution' and 'solvation' are two different processes, dissolution happens first when the solvents electric field goes into the crystal lattice and lowers the energy requires for ionization, then the ions are solvated by solvents. First step is all about the dielectrics and is about ionizing the species in the lattice, the second step is all about the polarity and becomes some annoying surface physics issues where even geometries can muddle things.

But I feel like the main issue with computers in water is that there are dissolved minerals in water that will cause all sorts of redoxs to happen.

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