# Tried charging a penny/dime battery. Yellow goo bubbled. What did I create?

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?

• Which coin was connected to which electrode? – vapid Oct 11 '16 at 7:12
• Vapid -- Sorry, I do not recall. – Robert G Oct 12 '16 at 19:37

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).