Over time, an iron nail reacts with water to produce iron oxide, or rust. which of the following is a signal that rusting has taken place?

Explanation:

When iron reacts with the moisture present in air then an oxide layer tends to develop on the surface of iron. This process is known as rusting of iron.

Chemical formula of rust is . This layer of oxide on the surface of iron is reddish-brown in color.

For example, when an iron nail rusts then color of the nail changes to reddish-brown.

This change is a chemical change as a new compound iron oxide has formed in this process of rusting.

Hence, we can conclude that a color change on the nail from silver to reddish-brown is a signal that rusting has taken place.