Science Buddies: "Ask an Expert"

Hello,
I am a grade 9 student doing a sciece fair project on removing the tarnish from a penny. i have found many resources, but am unable to find one key piece of information: why is it that when i add sodium chloride (salt) to another substance, such as lemon juice, the removal of tarnish increases? i have tryed looking it up, and i have a theory. my theory is that the sodium is positivly charged, and is even more positivly charged then the copper, but i have a problem with that hypothesis :chloride is negatively charged. is it that i have the wrong information, or is it that somehting happens to the chloride when i add it to an acid. Does the acid become more acidic? i need to present my science fair project using scientific formula's and terms to explain what happened to the penny.
I could also like to interview an expert on the subject. please respond, it would be alot of help for me.
~Jackie

This is a very interesting question: Remember that the Na and Cl in the compound NaCl are really ions (they are held together by ionic bonds, as opposed to covalent bonds). Now the acid in the lemon juice when combined with NaCl, frees up Cl- ions in the solution, which are highly corrosive toward copper oxide (which is tarnish). Ionization has no detrimental cosmetic effect on the copper base metal, and it leaves the base metal with a shiny finish.

Check out this site for reference:

http://www.finishing.com/4200-4399/4267.html