What Causes the ‘Rust’ on Coins Over Time?

Coins turn rusty because they react with air and water over time. Imagine the coin is like a hungry person, it wants to eat oxygen from the air, and when there’s water around, it eats even more. This makes the coin change color and feel rougher.

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Examples

A copper penny left on a windowsill turns green after months in the sun.
Coins in your piggy bank may look dull if they are left there for years.
Old coins in your wallet might have a rough texture from oxidation.

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Categories: Science · chemistry,oxidation,coins