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Apparently, electrical charge is transported by the ions dissolved in water, is this true?

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1 Answer 1

Yep. Pure water is an extremely bad conductor of electricity, it has very few ions. Water with an electrolyte (like NaCl) is a much better conductor of electricity; as the ions can migrate. Migration of ions is just like migration of electrons. If you place an imaginary surface inside the cell, there will be net negative charge crossing over to the positive terminal and vice versa. This is just like a current. Since there is net current inside, its conducting.

The equivalent conductance (a loco* chemistry concept) of a solution is simply the sum of the conductances of its constituent parts (Kohlrausch law). Here, $\Lambda$ denotes equivalent conductance of a portion of the solution, and $\lambda$ is the same for ions. Just a notation.

For pure water, $$\Lambda_{H_2O}=\lambda_{H^+}+\lambda_{OH^-}$$ Now, since the concentrations of $H^+$/$OH^-$ are small ($10^{-7} M$ at STP), the $\lambda$s and thus $\Lambda_{H_2O}$ are pretty tiny. For water with salt in it, we get $$\Lambda_{soln}=\Lambda_{H_2O}+\Lambda_{NaCl}=\lambda_{H^+}+\lambda_{OH^-}$+\lambda_{Na^+}+\lambda_{Cl^-}$$

Since $NaCl$ nearly dissociates completely, we get large $\lambda$s, and thus $\Lambda_{soln}$, which can be related to conductivity (in the aforementioned loco way).

So, pure/distilled water is an extremely bad conductor, while impure water with ions in it is a good conductor

* Loco because they assume a 1 m cell throughout, and don't keep the necessary $\text{m}^{-1}$ or whatever in their units. Due to this fixing of parameters, yes, we get that $\text{Area of plates}=\text{volume}$, which lets us relate it to concentration; but this gives us predictions for a specific case; when length of the cell is 1 m only. For some reasons these predictions are blindly applied to the general case. The whole thing gets confusing if you try to visualise it.

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Excellent, thank you. So it's not the electrons moving through the water but the ions moving to counter balance the charge of the electrodes (+ve ions going to an electrode with electrons and anions going to the electrode lacking electrons). So under AC this can occur forever, but under DC, at some point presumably, the current will eventually stop flowing once all the available ions have moved to either electrode. Is that correct? (and thanks for the foot note!... awesome :-) ) –  AJP Mar 5 '12 at 8:21
    
Hey @Manishearth do you have any thoughts on "... presumably, the current will eventually stop flowing once all the available ions have moved to either electrode. Is that correct?". Thanks. –  AJP Aug 6 at 15:02
    
@AJP yes. This takes time (forever) though, since there is a solid-ion equilibrium for the impurities, when the ions start disappearing ions are created from the solid. But it will try to maintain the equilibrium so the number of new ions will be much smaller. This process will go on, effectively to infinity since the equilibrium laws won't let the amount of solid or ions disappear completely. Practically, there will be some point when these numbers become too small, but again, practically this is very far off. –  Manishearth Aug 7 at 11:13

protected by Qmechanic Dec 9 '13 at 0:20

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