What effect will lemon juice have on a penny?

The acidity of the lemon juice will react with the copper in the penny, causing it to dissolve and turn a reddish-orange color. The chemical reaction that takes place is:

Cu(s) + 2H+(aq) + 2Cl-(aq) → CuCl2(aq) + H2(g)

In this reaction, the copper atoms in the penny are oxidized (losing electrons) to form copper ions, which then react with the chloride ions in the lemon juice to form copper chloride. The hydrogen ions in the lemon juice react with the electrons released by the copper atoms to form hydrogen gas, which bubbles away from the surface of the penny.