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I am attending a cram school and there was a question that goes:

Electrolysis decomposes chemicals by passing an electric current through a liquid or solution containing ions. Which of the following can be decomposed by means of electrolysis?
A. Water
B. Cream
C. Saltwater
D. Carbonated drink

My answer was C since the first sentence explicitly stated that electrolysis decomposes chemicals by passing an electric current through a liquid or solution containing ions, but they said that the correct answer is A. Also, I was thinking that water in itself is not conductive and therefore will not facilitate electrolysis by itself. Is the anwer really Water? And if so, why water and not saltwater?

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    $\begingroup$ That's an odd question, are you sure it's written like that? Pure water isn't conductive (very high resistance) but it can be subject to electrolysis: It's ionic product is 10e-14, meaning there are ions present. They are converted to H2 and O2. Saltwater can also be subject to electrolysis forming H2 and Cl2, and leaving behind NaOH. $\endgroup$ – IanC May 30 at 13:32
  • $\begingroup$ The question is written exactly like that and I was really baffled because I am very familiar with the chloralkali process which uses electricity and saltwater so I was certain that it should be saltwater. I, however, didn't know that pure water can also be subjected to electrolysis. $\endgroup$ – Jasper May 30 at 13:58
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    $\begingroup$ Scientifically speaking, that question does not make any sense, but it can be an approximation. What class level is it supposed to be? $\endgroup$ – SteffX May 30 at 14:05
  • $\begingroup$ It was meant to be a review for college entrance exams $\endgroup$ – Jasper May 30 at 14:08
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    $\begingroup$ It is a poor question written by an inexperienced person. Your logic and reasoning is perfectly fine. By that logic, carbonated drink can also be electrolyzed. $\endgroup$ – M. Farooq May 31 at 2:42
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What is the "correct" answer

You are asking why the answer key has water and not saltwater as an answer. Decomposition is defined as:

A reaction where a single compound breaks down into simpler compounds.

For salt water, the salt reacts at one electrode, and the water at the other, and you are making NaOH. For pure water, you are splitting water into the elements. Neither process matches the definition of decomposition that I quote exactly, but pure water is the better fit.

In your question, you write:

I was thinking that water in itself is not conductive and therefore will not facilitate electrolysis by itself.

The half reactions could be written as:

$$\ce{2H2O + 2e- -> H2 + 2OH-}$$ and

$$\ce{H2O -> 1/2 O2 + 2H+ + 2e-}$$

So there are protons produced at one electrode and hydroxides at the other (You could also have protons be reduced at one electrode, and hydroxide oxidized at the other - the net effect is the same). The protons are very mobile in water and will quickly diffuse and neutralize the hydroxide. This takes care of the charge transport through the solution.

Is this a good question?

No, but most multiple choice questions aren't.

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