The chemical reaction between the acidic content of the potato and the positive and negative electrodes occurs because the potato contains phosphoric acid (H3PO4), which helps conduct an electro-chemical reaction
      between the zinc and the copper. Zinc is an active metal, which means that it reacts strongly and or quickly with other substances, and are very reactive and sensitive with other substances. Active metals such as zinc have only a single electron in their outermost shell, which is very sensitive to being used to form a complete shell in some other atom.  The potato's phosphoric
      acid's active ingredient of phosphorus is positively charged hydrogen. The active metal which is zinc in this case, reacts in where the electrons between the zinc and the phosphoric acid are transferred between one another.
      The active metal, zinc, is oxidized and the phosphoric acid is reduced to hydrogen gas. This change is chemical as well as physical because you can see the acid bubbling out around the zinc because of the transfer of the
      electrodes. The copper substance's reaction to this is to deplete the electrons from the copper and to attach itself to the hydrogen ions in the phosphoric acid, which was formed when the electrons from the zinc were oxidized and
      the acid around the zinc was changed to hydrogen gas. This, in turn creates a current between the acid, the copper, and the zinc, creating energy