I tried the simple, routine experiment of separating water into H and O. I don't have lab equipment available, so I simply attached two paper clips to a 9V battery, and submerged them in water with no salt or other solute in it. I have heard of it done like this before, and thought I might recreate it. Contrary to what I expected, one terminal (the negative one), gradually turned black or dark gray, while the other let out many bubbles. I was surprised but assumed this didn't mean much, so I put a vial over the terminal with the bubbles, and waited a while.
After about 15 minutes, I checked on it and the water was notably green... there was also some solid matter on the top of the water, close to the battery.
My theory is that my paper clips are made of Copper, which dissolved to form Copper(II) ions which formed a green salt.
But still, I want to know what happened, why the - terminal turned black, and why there were no bubbles at it.