I set up an electrolysis experiment to show my kids and wanted to explain to them what is happening.

My setup has a penny as the anode and graphite for the cathode. The solution is water with baking soda mixed in.

I see bubbles at each of the electrodes which I assume are hydrogen and oxygen bubbling out. I also see some cloudy substance coming off the penny, which I assume is some sort of copper compound, but I'm not sure.

Can someone help me figure out what's going on in the setup at the two electrodes?

Edit : corrected the electrode descriptions. I earlier stated penny as the cathode and graphite as the anode.

  • $\begingroup$ For a demo like this, one should rather use graphite electrodes. There is a reason why reactive metals are not employed in electrochemistry as electrodes. $\endgroup$
    – AChem
    Aug 9, 2020 at 3:45
  • $\begingroup$ What voltage did you apply? $\endgroup$
    – Karsten
    Aug 9, 2020 at 21:46
  • $\begingroup$ I had a 9V battery attached. $\endgroup$
    – bluknite
    Aug 10, 2020 at 7:31

1 Answer 1


Yes, I agree you are getting hydrogen an oxygen as you expected.

However, the copper anode is undergoing anodic corrosion and forming CuO (which is characteristically black) and possibly also some Cu2O (yellow or red, depending on the size of the particles, where the color combination could be described as orange).

Now, in the presence of HCO3- and O2, a naturally occurring mixed salt, copper patina (which forms on the copper roofs of houses) could also be forming. Wikipedia on patina, to quote:

patina is a coating of various chemical compounds such as oxides, carbonates,... formed on the surface during exposure to atmospheric elements (oxygen, rain, acid rain, carbon dioxide,...

Patina has been described as a blueish-greenish naturally forming on a copper surface (here, your copper penny electrode).

A closely related version of your electrolysis experiment has been previously discussed elsewhere.

  • $\begingroup$ The OP said that a graphite anode was used; the copper penny was the cathode. "Cloudy" substance coming off the penny was not described as colored - but perhaps so faint as to be undetectable. And yet, oxidation should not be occurring at the cathode. $\endgroup$ Aug 9, 2020 at 13:30
  • $\begingroup$ The patina did show up after a while. I also noticed that the penny was pretty eaten into at the end. So I suspect that some of the copper made it into the solution - perhaps as the white cloudy stuff in the solution. Could this be a copper + carbonate compound? I also just realised that I had the electrodes backwards (copper anode and graphite cathode) $\endgroup$
    – bluknite
    Aug 10, 2020 at 7:29

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