Which type of change involves the formation of new substances?

The formation of new substances is characteristic of a chemical change. During a chemical change, the original substances undergo a transformation that results in new substances being created with different chemical properties. For example, when iron rusts, it reacts with oxygen in the presence of moisture to form iron oxide, which is a different substance than elemental iron. This process involves breaking and forming chemical bonds, which leads to the creation of these new substances.

In contrast, a physical change does not alter the substance's chemical identity but may change its state or appearance, such as when ice melts into water. A thermal change typically refers to changes in physical states caused by temperature variations, and an electromagnetic change would involve changes in energy due to electromagnetic interactions, neither of which specifically create new substances. Thus, the defining factor for a chemical change is the production of different substances, making it the correct answer for this question.