Which of the following is a sign of a chemical change?

A change in color is a significant indicator of a chemical change because it often suggests that new substances have formed as a result of a chemical reaction. This may occur due to the breaking and forming of chemical bonds, where reactants are transformed into products that have different properties, including color. For example, when iron rusts, it changes from metallic iron to iron oxide, a process that involves a color change.

In contrast, the freezing of water, boiling of water, and mixing two liquids are examples of physical changes. In these scenarios, the substances involved do not undergo any change in their chemical identities; rather, they only change physical states or combine without any chemical reaction taking place. For instance, when water freezes or boils, it remains H2O and retains its chemical structure, regardless of whether it is in solid, liquid, or gaseous form. Therefore, observing a change in color is a key hint that a chemical change has occurred.