Why does lemon juice clean a penny?

Lemon juice can clean a penny due to a chemical reaction that takes place between the acid in the lemon juice and the copper oxide layer on the surface of the penny. Copper oxide is a dark, tarnished layer that forms on copper when it is exposed to oxygen. The acid in the lemon juice reacts with the copper oxide to form copper citrate, which is a water-soluble compound. This compound dissolves in the lemon juice, removing the tarnish from the penny and leaving it shiny and clean.

The chemical reaction that takes place can be represented as follows:

CuO (copper oxide) + H3C6H5O7 (citric acid) → Cu(C6H5O7)2 (copper citrate) + H2O (water)

In addition to lemon juice, other acidic substances such as vinegar or ketchup can also be used to clean pennies.