Direct current is often used in electrolysis and because of the alternating nature of AC, it's not great for electrolysis. Tap water, however, conducts AC really well.

But why is that? Why does alternating current flow through water well following the Ohm's law and why does DC act like a diode; water passes DC through after a certain voltage point and from there on it conducts it just fine?

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    $\begingroup$ Do you have some reference to document these observations? For example, the current versus voltage in DC and AC. $\endgroup$ – nasu Apr 10 '17 at 13:22
  • $\begingroup$ Homogeneous water even with if an electrolyte surely does not act as a diode for a diode conducts differently upon polarity change of bias (at least this the conventional definition of what a diode is). All diodes (vacuum or solids-state) are materially asymmetrical and that manifests in their electric behavior. $\endgroup$ – hyportnex Apr 10 '17 at 13:41
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    $\begingroup$ @nasu Only my own measurements which should represent the effect. They might not be really accurate given the equipment used. i.gyazo.com/9d39a1afb1239ad9c5347fedc0bae683.png $\endgroup$ – Sami Apr 10 '17 at 13:41
  • $\begingroup$ @hyportnex My physics professor compared the effect to a diode, however. Neither was I convinced it works that way, but the measurements I took linked above, show that DC doesn't flow similarly to AC. $\endgroup$ – Sami Apr 10 '17 at 13:45
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    $\begingroup$ For the AC regime you will have a current even if you have nothing (air) between electrodes. Have you tried to estimate the capacitive reactance of your setup? I mean, pure water is a good dielectric so a capacitor filled with water may have significant capacitance. $\endgroup$ – nasu Apr 10 '17 at 14:40

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