How are "reactive" substances characterized in TDG classifications?

Reactive substances are characterized in TDG classifications as materials that can undergo chemical reactions, which pose risks if they are not handled properly. These reactions can result in hazardous conditions, such as fire, explosion, or the release of toxic gases. As such, understanding the reactivity of these substances is critical for ensuring safe transportation and handling.

This classification highlights the importance of proper training and safety measures when dealing with reactive materials, as their potential to engage in dangerous reactions requires careful management to prevent accidents. In contrast, the other options incorrectly suggest that reactive substances are either completely safe, incapable of reacting, or always stable, which underestimates the risks associated with handling these types of materials.