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In attempting to reveal the copper under a quarter, I used alligator clips to attach the quarter and a clump of copper wire to opposite sides of the battery. I put the electrodes in a solution of tap water and NaCl.

As the reaction occurred, the solution quickly turned into an icky (for lack of a better word) green, the water became very unclear and some cruddy particles rose to the top. Also, gas evolved from the copper electrode (which I suspect is H2). The coin blackened in this process and I took the quarter out and washed it.

Next, I reversed the polarity and the coin became the more expected copper color. As this happened, gas rose from the coin instead.

While I don't understand what occurred, my concern is the green sludge that was produced. I read solid nickel surrounds the mostly copper quarter coin. If the gas is really H2, then there must be hydroxide ions produced. If this is true, then I suspect the sludge could contain nickel(II) hydroxide. Is this possible? The "sludge" was a dark dollar green and was flaky, unlike dry nickel(II) hydroxide. It really looked like sewage more than anything else. I did not get a picture because I dumped it out before I thought of taking one (I suspected it to be dangerous). Are the conditions I described enough to get an idea of what it was?

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