Final answer:
The hazard label 'Spontaneously combustible' is used for materials that can ignite on their own when in contact with air, differing from flammable gases and solids. Such materials are a subcategory of characteristic hazardous wastes and may be classified under special hazards in the NFPA hazard identification system.
Step-by-step explanation:
The hazard label "Spontaneously combustible" refers to materials that are liable to spontaneously catch fire as a result of their interaction with air without requiring an ignition source. This characteristic is different from flammable gases like methane (CH4), which combine with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O), releasing heat after ignition. Similarly, the label distinguishes these materials from flammable solids, which are also capable of catching fire but typically require an ignition source.
Characteristic hazardous wastes are defined by properties such as ignitability, corrosivity, reactivity or toxicity, and the Environmental Protection Agency (EPA) has specific classifications for these materials. Materials labeled as spontaneously combustible may pose a significant fire hazard due to their propensity to catch fire in normal environmental conditions without an external ignition source. The hazard identification system created by the National Fire Protection Agency (NFPA) includes a white diamond that indicates if a substance is an oxidizer, reflecting its potential to facilitate combustion even in the absence of air/oxygen.
An example of how oxygen supports combustion can be seen when a glowing splint is introduced to pure oxygen gas compared to air, where the concentration of oxygen in pure gas causes the splint to burst into flame, showing the reactivity of oxygen in combustion reactions.