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Coca-Cola leaves the penny bright and shiny.

Coke does this by corroding the copper oxide layer that forms naturally through exposure to the air over the penny's copper plating. While the oxide layer is stable and prevents further reaction of the copper plating, it obscures the shiny copper plating to give the penny a darker, dull appearance.

Coke is able to remove the oxide layer on pennies because of its acidity (Coke has a pH of approximately 2.5, similar to lemon juice). Acidity gives it corrosive properties. To be more specific (and a tad uncertain), the phosphoric acid in Coca-Cola likely reacts with the copper oxide layer to remove it.

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14y ago

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