Re: Why does the tarnish on a pennie dissapear when you put in lemon juice?

Pennies are made out of copper, one of the many metals that react with 
oxygen. That's why pennies get dull when exposed to air: the copper in 
them forms a new chemical compound by bonding to oxygen. If the penny has 
been on the street for a while and has colected a good deal of moisture, 
then it will continue to react with carbon dioxide and water and form the 
same green stuff that you see on old bronze statues.

Now both the dull stuff , what you called tarnish ( copper oxide), and the 
green stuff don't dissolve in water. But both dissolve in acid, which is 
why the lemon juice, which contains acid, got rid of the tarnish.