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I filed one side of a penny to reveal the zinc; then I bent the coin and dropped it into a cup of white vinegar. Shortly afterward, many tiny bubbles were forming on the zinc side. Later on, I noticed a black residue on the zinc side. What might be the reaction that occurred? [I believe that the gas was either H2 or CO2 gas. I also believe that the black residue was either CuO or carbon] {Zn + Cu + H ion + CH3COO ion --> ???}

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    $\begingroup$ The zinc reacted with the acetic acid in vinegar, with hydrogen gas being evolved. The black stuff is not carbon or CuO. Just some kind of crud. $\endgroup$
    – Ed V
    Commented Dec 13, 2022 at 2:40

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Copper surface was probably a little bit oxidized into $\ce{CuO}$. Dipped into vinegar, this oxide is quickly dissolved by the acetic acid, producing $\ce{Cu^{2+}}$ ions in solution $$\ce{CuO + 2 CH3COOH -> Cu^{2+} + 2 CH3COO^-}$$Then, zinc may react with these copper ions, producing copper metal which looks black if the dimensions of the grains are small enough. $$\ce{Cu^{2+} + Zn ⟶ Cu + Zn^{2+}}$$

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  • $\begingroup$ Thank you, Maurice. $\endgroup$ Commented Dec 19, 2022 at 16:35

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