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When performing electrolysis at home, I usually used salt water as electrolyte. The problem is, that this method also produces chlorine and chlorine is aggressive to both humans and electrical equipment.

I was thinking of creating pure hydrogen and oxygen by using distilled water. I was taught that water is not conductive.

But since water has "positive" and "negative" part (which allows for it to dissolve polar bonds, doesn't it?). Couldn't I just tear the molecules apart if I use electrical field strong enough? If I can, how high the voltage must be?

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    $\begingroup$ I have Buder HS-72 Hydrogen Water Generator at home. I put distilled water in it and it still works! When I set it to cleaning mode it produces ozone and oxygen but not chlorine gas since I use distilled water only. As ozone has a very distinct smell and I could sense it I have therefore empirical evidence that yes, you can perform electrolysis without electrolyte. I can't exactly say how the machine technically works but the mechanism probably relies on having a bunch of conductive metal plates very close to each $\endgroup$
    – Hyena
    Commented Jan 25, 2023 at 20:34
  • $\begingroup$ This Buder HS-72 apparatus does not produce hydrogen gas. It produces a hydrogen solution in pure water. It means that the concentration of $\ce{H2}$ in water is extremely low. The maximum solubility of $\ce{H2}$ is $1.4$ mg per liter. Such a concentration is not far from the concentration of $\ce{H+}$ ions in aerated pure water (pH = $5.6$). So it could be admitted that water is electrolyzed as such a low amount. If some electrolyte had been added, the electrolysis could produce more $\ce{H2}$, but this would be included in a mixture of $\ce{H2 + O2}$, which is explosive ! $\endgroup$
    – Maurice
    Commented Jan 25, 2023 at 22:39
  • $\begingroup$ You could try using a different salt to make the water conductive. sodium carbonate, for example: no possibility of chlorine emission there. $\endgroup$
    – matt_black
    Commented Jan 26, 2023 at 20:55
  • $\begingroup$ @matt_black That's actually what I eventually figured out. I would put baking soda in an oven to turn it in sodium carbonate and than use that. $\endgroup$
    – Tomáš Zato
    Commented Jan 27, 2023 at 10:14
  • $\begingroup$ Interfacial water is more conductive than bulk due to microcrystal structure from surface nucleation which is why it works on "plates very close to each other" $\endgroup$
    – Devin Peterson
    Commented Jan 28, 2023 at 18:40

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Under any low voltage, sufficient to decompose water in conducting solution, the electrolysis of pure water is going as well, as it's allowed thermodynamically. It's just very slow because the amount of product per time depends on amount of charge per time, i.e., current, not voltage.

In the case of water, you can't significantly raise the current by raising the voltage, because you sooner reach dielectric breakdown than any sufficient current. Instead, you better invent some nanostructure to minimize space between electrodes and maximize their area :)

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  • $\begingroup$ You mean, for example, two copper plates very close to each other? Or do you mean something that is really "nano", that is, too small to design at home? $\endgroup$
    – Tomáš Zato
    Commented Sep 29, 2016 at 13:58
  • $\begingroup$ Not only close, but the surface must be maximized to get as much current as possible. Don't think that "nano" is too small to design at home. I don't know how to arrange pure water decomposition at home, but there are very porous substances like medical charcoal or silicagel and the ability (at least theoretical) to coat or impregnate them. $\endgroup$
    – sa7
    Commented Oct 3, 2016 at 14:35
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If I can, how high the voltage must be?

Not possible at home.

For obtaining pure hydrogen and oxygen it is still required to use an electrolyte. However, there is some amount of them that are not electrolyzed, allowing to electrolyze only water. I'd try sodium/potassium hydroxide or sulfate.

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  • $\begingroup$ Even if it's not possible at home, it would be interesting. I was thinking, however, that at certain voltage a spark will go through water instead of elecrolysis. $\endgroup$
    – Tomáš Zato
    Commented Jun 24, 2014 at 17:12
  • $\begingroup$ Does sodium hydroxide get electrolyzed? $\endgroup$
    – Tomáš Zato
    Commented Jun 24, 2014 at 20:22
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    $\begingroup$ @TomášZato 1) yes, it would. However, it occurs in different way, see 'Electrical breakdown' part of 'Insulator (electricity)' on the wiki. 2) Electrolysis of $NaOH$ solution will produce hydrogen and oxygen and may be a trace amount of $H_2O_2$ will be produced as well. If it traps some amount of $CO_2$ forming carbonates, some amount of peroxocarbonate may be produced as well. Good thing, both will stay in solution, not producing any harmful gases. Theoretically speaking, sodium may be obtained by electrolysis of water solutions, but it requires mercury electrode. $\endgroup$
    – permeakra
    Commented Jun 24, 2014 at 21:50
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I've ran a 240v supply through a container of de-ionised water and even rain water to connect up to a light bulb. The light bulb never worked. I even placed my finger in the waters and that didn't get a shock or trip my rcd. So, imho, water without impurities does not conduct electricity and it's only the impurities that do (don't put your finger in water that has impurities!!) So if water isn't conductive how can it be broken down into the gases by electrolysis? Well, put simply, it can't, people always add an electrolyte to the water but it is the electrolyte that is being broken down, not the water. Hope that clarifies your question.

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    $\begingroup$ The electrical conductivity of pure water at 25 °C is 0.055 µS/cm. This value may be very low, but it is not zero. $\endgroup$
    – user7951
    Commented Jun 17, 2018 at 13:20
  • $\begingroup$ Try with interfacial water, that is water adsorbed onto strongly charged surfaces like a plate, works better on anode $\endgroup$
    – Devin Peterson
    Commented Jan 28, 2023 at 18:43

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