Which symbol indicates unusual reactivity with water in the NFPA special hazard symbols?

The symbol that indicates unusual reactivity with water in the NFPA special hazard symbols is represented by the letter 'W.' This specifically stands for a warning regarding the material's water reactivity. It is crucial in laboratory settings to be aware of substances that can react violently or produce hazardous gases when in contact with water, as this can pose serious safety risks during experiments or emergencies.

In contrast, other symbols have different meanings. For instance, 'OX' signifies an oxidizer, which can enhance combustion or cause a fire in the presence of combustible materials. The letter 'R' indicates reactive materials that may be unstable under normal conditions, although it does not specifically address reactivity with water. 'A' typically denotes an asphyxiant, not focusing on reactivity concerns. Understanding these symbols and their implications is fundamental to maintaining safety in environments where flammable gases and other reactive chemicals are utilized.