Where would you expect to find the centre of gravity of a plastic ruler?

You would normally find the center of gravity in the middle of the ruler. Example: the ruler is 1 foot long, the center of the gravity would be a 6 inches.
 

The chemical reactions lab we did is typically called a "copper plating lab,  because the acid solution cleaned the pennies and made the copper look shinier;   the acid solution cleaned the paperclips and made them look like pennies;   &   the copper from the pennies was removed, and then added to the paperclips.