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I keep reading that solutions are electrically neutral, but I've never read an explanation. I've read after the fact justifications, such as one problem which asked me to calculate the force of repulsion between two beakers with a tiny, tiny bit of excess charge (it ended up being over 10,000 pounds).

So why are solutions electrically neutral? Not interested in after the fact explanations like the above example.

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  • $\begingroup$ What do you mean by "after the fact explanations," exactly? Does an explanation based on Coulombic repulsion/attraction qualify? Or the accumulation of potential energy resulting from buildup of like-charged particles and/or separation of oppositely charged ones (and hence, the work done by, e.g., an E-field necessary to produce and/or maintain such an arrangement)? $\endgroup$ – Greg E. Jun 21 '14 at 3:14
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    $\begingroup$ it will be more useful if you give why you think they aren't or how can electrical neutrality of solution be disturbed ; because we never added charge externally, we just added salts, for an example thus sustaining it to be neutral. $\endgroup$ – RE60K Jun 21 '14 at 4:41
  • $\begingroup$ Possible duplicate (physics.stackexchange.com/questions/62181/…) or(chemistry.stackexchange.com/questions/4805/…) $\endgroup$ – Freddy Jun 21 '14 at 5:32
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    $\begingroup$ Take a look at this question, too. $\endgroup$ – Nicolau Saker Neto Jun 21 '14 at 12:34
  • $\begingroup$ @GregE. I mean that the Coulombic repulsion/attraction problem I read in my textbook was like saying humans aren't 100 feet tall because ceilings aren't 100 feet high. In other words it seemed to have cause and effect reversed. $\endgroup$ – Dissenter Jun 21 '14 at 12:39
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I think there is a misunderstanding here about the difference between explanations and observations.

The fact that two beakers of aqueous solution do not have a measurable force between them is an observation that proves the solution has no (large) charge. Calculating a hypothetical force due to a charged solution and finding that it is much larger than what we observe is not an after-the-fact justification, it is a falsification of a hypothesis by an empirical observation.

The explanation is that no matter what the ionic compound that you started with was, it was electrically neutral. Therefore, the solution must still be neutral.

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  • $\begingroup$ Are macroscopic solid ionic compounds really neutral though, or is that an idealisation? I don't think it's possible to make an argument that any real object must be exactly electrically neutral, just that it has to be neutral to within a good approximation. $\endgroup$ – Nicolau Saker Neto Jul 25 '14 at 20:58
  • $\begingroup$ So why must the ionic compound we started with be neutral? Again, I understand the stability arguments - i.e. isolating ions means we'd also be isolating electrons. So I guess our not being able to isolate electrons is an explanation rather than an observation. Thanks! $\endgroup$ – Dissenter Jul 25 '14 at 21:34
  • $\begingroup$ @NicolauSakerNeto I agree, we can "charge" glass by rubbing it with a silk cloth. Can we similarly add charge to a solution? $\endgroup$ – Dissenter Jul 25 '14 at 21:36
  • $\begingroup$ @Nicolau - let's put it this way: They are as neutral as any macroscopic compound can be. $\endgroup$ – thomij Jul 25 '14 at 22:42
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It is more acurate to say that they aren't very far from being electrically neutral. If you take a charged sphere and drop it into a beaker of some electrolyte sitting on an insulating counter, the result is a charged solution, albeit slightly.

Also, in spite of your amusing ceiling analogy (it made me chuckle), the argument about electrostatic attraction and repulsion does justify why solutions (and solids) can't be too far from being electrically neutral. The flaw in the analogy is that humans don't need ceilings to exist; but you can' t have two charged beakers without a force between them. If each has a charge of 1 C -- which is a mere $10^{-5}$ times the charge of a mole of electrons -- and they're sitting 1m apart the force between them is about a million tons -- clearly this situation won't last very long!

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Because the solute is neutral when its solid so it must me neutral when it's aqueous.

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