Which of the following hazards is classified under chemical hazards?

The classification of chemical hazards primarily relates to substances that pose risks due to their chemical properties. Explosive substances fall into this category as they can react vigorously, potentially leading to explosions if not handled properly. These substances often involve reactions with oxygen or other chemicals that produce rapid gas expansion and heat, which can be extremely hazardous in various environments, particularly those involving storage and transport.

In distinguishing between the other options, nuclear waste pertains to radiological hazards rather than chemical ones, as it consists of radioactive materials. Asbestos, while a significant health hazard due to inhalation, is classified as a physical hazard rather than a chemical hazard, primarily affecting respiratory health. Lastly, heavy metals are categorized more specifically under the broader bracket of metal hazards; while they are indeed toxic and can have significant health effects upon exposure, they do not fit neatly within the chemical hazard classification like explosive substances do. Therefore, explosive substances are correctly recognized as a type of chemical hazard due to their inherent properties and potential for dangerous reactions.