What does the yellow diamond on the NFPA 704 fire diamond represent?

The yellow diamond on the NFPA 704 fire diamond represents the reactivity of a substance. Reactivity refers to how likely a chemical will undergo a chemical reaction and potentially release energy or create hazardous conditions. The yellow color indicates the degree of hazard associated with the substance's reactivity, with higher numbers indicating a greater potential for reactive hazards. This information is essential for emergency responders and others handling hazardous materials, as it allows them to understand and mitigate the risks associated with the substance.