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When I was a young child, I applied 9 volts across an electric cell I made using a penny, a dime, and a salted piece of paper napkin wet with saliva. Immediately, an unknown mustard-yellow liquid began forming and bubbling from in between them. It discolored the coins.

What reaction took place? What was the yellow stuff?

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    $\begingroup$ Which coin was connected to which electrode? $\endgroup$ – vapid Oct 11 '16 at 7:12
  • $\begingroup$ Vapid -- Sorry, I do not recall. $\endgroup$ – Robert G Oct 12 '16 at 19:37
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You created an electrolytic cell. The reason why I have asked you in the comments about electrodes is to determine which coin served as a cathode, and which as an anode because this is crucial for the electrochemistry that took place in your experiment. Anyway, let's first discuss what happens if you electrolyze a solution of sodium chloride with copper electrodes and allow the solutions near the electrodes to mix (I was doing it this as a kid I remember). On the anode (the electrode connected to '$+$' on the battery) copper gets oxidized to its $+1$ oxidation state:$$\ce{Cu->Cu+ +e-}$$ (I will skip the reactions involving chlorine and all other reactions that are of no interest to us). On the other electrode (cathode, '$-$' on the battery) hydrogen ions that come from water dissociation: $$\ce{H2O->H+ + OH-}$$ are reduced to gaseous hydrogen:$$\ce{2H+ +2e- ->H2}$$ leaving out the $OH^-$ ions. When the solutions surrounding each electrode are mixed, Copper(I) hydroxide forms:$$\ce{Cu+ +OH- ->CuOH}$$ which instantaneously decompose to copper(I) oxide:$$\ce{2CuOH->Cu2O +H2O}$$ which is insoluble and yellow-orange in color. You can see this here (but it is better to do it yourself!). I believe that the 'yellow goo' you obtained in your experiment is the copper(I) oxide. This suggests that the penny served as an anode and the dime as a cathode (although both coins contain copper, so maybe it doesn't really matter which coin was which electrode).

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  • $\begingroup$ Some of the color can also come from nickel and copper chloro- and aqua- complexes, they have a yellow-green coloration. $\endgroup$ – vapid Oct 13 '16 at 9:55
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Hahaha, I had to translate for myself what a penny and what a dime is. Always the same for me; Fahrenheit, miles, ...

A penny is a copper-plated zinc coin. A dime is a nickel-plated copper coin. Knowing this, I would assume that the yellow color arose from the nickel content. If it would be blue, it would have more likely been copper. The goo. Such a structure often occurs with proteins, polymers, and similar compounds. It's most likely that the bacterial farm on the coins you used, lead to the formation of this goo. It's impressive how many bacteria and what thick biofilms you can find on a coin. Additionally, your saliva also contains a multitude of living organisms and their remains.

Therefore, you should definitely reproduce this experiment! Clean the coins beforehand using ethanol, acetone, and water while scrubbing. Polish them with a linen cloth and not with sand paper. Otherwise, you would damage the plated metal film and surface the underlying metal (makes things now complicated). And don't spit in it yet! :-)

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