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I am a new teacher teaching science to grades 6-8. In 8th grade, we have been exploring chemical and physical changes. They recently ran an experiment where they sanded the edges of pennies minted after 1985 and submerged them in white vinegar along with a 2x2 cm square of aluminum foil. They observed the bubbles forming on the edges of the pennies and saw nothing happen with the foil, which is what I expected would happen. I thought the eposed zinc on the eges of the pennies would react with the acetic acid and release hydrogen gas, and the aluminum foil would't react.

We left the pennies for a week or 10 days; I forget exactly, because I wanted to the students to see the precipitate form. To my surprise, a black preciptate formed, but many of the cups also produced white crystals. So we are running a new experiment with sanded and unsanded pennies, foil, and vinegar, in all the different possible combinations to find out which combination of materials forms white crystals.

That's been going about 10 days, and it looks to me like the pennies with the sanded edges form black precipitate and white crystals, and I am wondering if anyone can tell me what the chemical reactions are that I am observing. Are both solids zinc compounds? Or is one of them a copper compound? Maybe the black is copper oxide and the white is a zinc salt? I would love to know the correct answer since it will flow nicely into our next lesson on balancing chemical reactions.

Thanks!

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As a quick follow-up, it is possible that the cups we used were previously contaminated with salt and not washed properly. I am not sure. I just set up a new experiment with sanded and unsanded pennies, salt and no salt added to the vinegar. They are bubbling away right now.