What does the yellow square in the NFPA Diamond indicate?

In the NFPA Diamond, the yellow square represents Reactivity or Instability hazards. This indicates the potential of a material to undergo a sudden or rapid change under certain conditions, which could lead to dangerous reactions. The rating within this square conveys how reactive the substance can be; for instance, a higher number indicates a greater potential for dangerous reactivity. Understanding this aspect is critical for safety in laboratories, especially when working with flammable gases or other hazardous materials, as it helps in assessing the safe handling and storage protocols needed to mitigate risks associated with chemical reactions.