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There's a disagreement between me and my high school chemistry teacher regarding the answer to this question. I'm not convinced by his argument and neither is he convinced by my argument.

The problem goes as following

What balances charges that may build up as reduction and oxidation occur in a voltaic cell?

a. the salt bridge.

b. the electrolyte solutions

c. one of the half-cells

d. the moving electrons

We both agree that the word "charges" in the question refers to electrons. I argue that the answer is "a. the salt bridge". The salt bridge allows ions to pass to between the half-cells which prevents the build-up of charge on one of the half-cells due to the electrons, allowing the reaction to continue. So the salt bridge counter-acts or balances the charges. My teacher claims that the answer is "d. the moving electrons". He argues that moving the electrons balances the charges (I can't really phrase his argument).

My understanding of "balances the charges" is "counter-acts the effect of the charges". Perhaps this is where I just don't understand the question or my teacher. How can the movement of the electrons balance the charges or, in my understanding, counter-act the effect of the charges when it, in my understanding, is what creates the effect of the charges.

What is the correct answer and why?

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    $\begingroup$ A bridge is optional, need not to be present if half cells share the electrolyte. Electrons are not present in electrolytes but ions. Free electrons reduce water in less then millisecond. $\endgroup$
    – Poutnik
    Commented Oct 20, 2022 at 6:07
  • $\begingroup$ Also, electrons, coming and leaving, disbalance charges. $\endgroup$
    – Poutnik
    Commented Oct 20, 2022 at 6:32
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    $\begingroup$ What a terribly asked exercise! You're both kind of correct and kind of wrong at the same time. The electrons from the oxidation half cell just move to the reduction half cell, answer d. At the same time, ions must travel the other way. This could be via a salt bridge, but that doesn't make it the correct answer. $\endgroup$
    – Martin - マーチン ♦
    Commented Oct 20, 2022 at 6:33

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Let us clarify a couple of fundamental aspects. In a functional galvanic/voltaic cell there is always a very small charge imbalance at the electrode solution interface, this is why voltage difference develops between the two electrodes. However the bulk of solution is electrically neutral. This bulk neutrality is indeed achieved by a salt bridge. If you go deeper, the salt bridge can also have a slight charge imbalance due to different speeds of the anion and the cation in the gel. So your argument is correct about salt bridge in a general sense.

The conceptual problem with the question is the fact that none of you are specifying the part of the galvanic cell. Are we talking about the bulk solution, are we talking about the electrode-electrolyte interface or are we talking about the external circuit?

It just reminds me of the famous but really old school poem, The Blind Men and the Elephant, when six keen scholars went on to explore an elephant. One holds the legs and thinks that the elephant is a trunk. The second one holds the broad side and calls it a wall, the third touches the tusks and thinks that the elephant is nothing but a spear ...and the fight continues...

And so these men of Indostan, disputed loud and long,

each in his own opinion, exceeding stiff and strong,

Though each was partly in the right, and all were in the wrong!

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