Megan left her bicycle outside on the patio while she went on vacation for several weeks. When she returned, Megan discovered a reddish substance, rust, on the handlebars of her bicycle. The rust is a

Megan left her bicycle outside on the patio while she went on vacation for several weeks. When she returned, Megan discovered a reddish substance, rust, on the handlebars of her bicycle. The rust is an example of a(n) __________.