What happens to resistance of tap water as voltage is increased? In recent days I have done a few experiments measuring the current of water as it goes up from 9 volts up to 36 volts, and following Ohms law to convert it to resistance. And I discovered a very interesting trend. In between 9 and 18 volts, there is a massive drop in resistance (by around a 40% reduction) but then as I go up to 27 volts, its a 5% reduction, and is even less of a reduction when reaching 36 volts. I've done this experiment a few times and this has continued to happen. This is it visualised on a graph;


I am curious to know why this happens, why there seems to be a reduction as I go from 9 to 18 volts, yet the reduction seems to reduce at 27 volts and reduce further at 36. Is there a reason that? And as I go further up the voltages (don't want to test with higher), does this continue with the reduction in resistance continuously reducing, and if not at what voltage does it change?
Specifically am asking why this is happening and what happens when the voltage gets higher. I kinda want to know what resistance can I expect at around 240 volts
 A: You are electrolytically decomposing your test electrodes. They must be made of platinum to prevent this effect.
A: You are forcing a linear fit through a few data points. The actual I-V curve for an electrolitic cell is non-linear. It may fit an exponential on some range but it may also show a peak at some specific voltage. So, it is a non-linear (non-ohmic) "resistor". You can divide the voltage by current for each data point and call it "resistance" but there is no reason to expect a linear dependence on voltage and even less a constant value. The reason for this is that there are regions in the cell (near the electrodes) where the current is driven by diffusion so it is not a drift current. The thickness of the diffusion layer(s) changes with the voltage applied. The actual shape of the curve may depend even on how fast you change the voltage applied to the cell. As well as on the electrode material and the concentration of ions in the solution. So, there is really no mistery that different experiments show different results or that your curve does not have a constant slope.
