I was conducting a simple experiment at home on electrolysis. Obviously, when you build a circuit with a 9V battery, a lamp, and put two electrodes in water you measure a potential difference of ~9V on the two electrodes while the potential difference on the lamp is 0 (really close to 0). Then as I kept adding salt in water, I first measured that the potential difference across the lamp got higher and higher (reaching 6V), then suddenly, as I got from 40g /250ml of salt in water to 50g /250ml, the potential difference across the lamp fell to 4V.

Why is that? Maybe is it because of the ion association occurring in water as the concentration of salt gets higher?

4.5 V lamp. 8.6 e.m.f battery.

  • $\begingroup$ What was the voltage across the electrodes at 40g and 60g of solute? $\endgroup$
    – bpedit
    Sep 27 '16 at 19:12
  • $\begingroup$ what kind of lamp? $\endgroup$
    – MaxW
    Sep 27 '16 at 19:24
  • 2
    $\begingroup$ It sounds like you might be witnessing corrosion on the electrodes, which I would expect if you allowed electrolysis to continue. You should try different concentrations with new (or at least vigorously cleaned) electrodes and see if the results are similar. $\endgroup$
    – user7652
    Sep 28 '16 at 20:28
  • $\begingroup$ It would be good to clarify the exact experimental set up. Lamp as commented before but more importantly what were the electrodes made from? By salt, I am assuming culinary table salt, which is mostly sodium chloride, but often contain sodium iodide or potassium iodide and an anti-caking agent such as sodium aluminosilicate. Were your solutes fully dissolved? Were the solutions at the same temperature? What else did you observe? $\endgroup$ Feb 6 '17 at 8:05

Tungsten lamps change resistance as they warm up. At room temperature, the resistance of a small flashlight lamp (e.g. #222) might be 10 ohms when brightly lit, but only 1.5 ohms at room temperature.

At first, the tungsten filament acts like a short circuit, so most of the voltage of the battery is dropped across the series resistance of the electrolytic cell. Once enough current goes through the lamp, though, its resistance abruptly rises and more of the voltage is dropped across the filament.

If you use an LED instead of a tungsten-filament bulb, the change in voltage would be more linear as you add salt. Note that an LED will be effectively an open circuit until a minimum voltage is reached (~1.6 V for a red LED, ~3 V for blue or white LED), but then current increases linearly until ~20-50 mA, at which point some may burn out.

  • $\begingroup$ This does not explain why the voltage dropped at higher solute concentrations. $\endgroup$ Feb 6 '17 at 7:58
  • $\begingroup$ Yes it would The tungsten element got warm, and the resistance increased. At the same time, more solute was added. The observed drop in voltage was correlated with higher salt concentration. Correlation <> causuality. $\endgroup$ Feb 6 '17 at 13:42

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