The penny must have changed its color because an oxidation reaction. When we heat some metals, the trnasform into oxides, for exxample Zinc oxide ZnO or CuO.
Moreover, his compounds are colored molecules mostly brown or black colored compounds. First at all when we put the pennies into a acetic acid solution, a redox reaction wiil happen. The acetate is an oxidant and it will react with the Cu, Zn and Sn to reduce them. for example, if we have ZnSO4 in the plate and it will be heated, the Zn will suffer a redox reaction and will form Zn0. a metalic form. that´s why the color changes.
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