# Is tarnishing considered a physical or chemical change?

$\text{Tarnishing}$ is properly considered a chemical change.
Chemical change is characterized by the formation of new substances, and the making and breaking of chemical bonds. When a metal tarnishes, typically copper, brass, silver, aluminum, or magnesium, it reacts with atmospheric oxygen to form a thin oxide coating. When iron tarnishes, we call the phenomenon $\text{rusting}$. Some metals, particularly gold, are very resistant to tarnishing, and this is one of the reasons why gold is so valued as an ornamental metal.