What does the term pyrophoric mean?

The term "pyrophoric" refers to materials that can ignite spontaneously when exposed to air or moisture at room temperature. Therefore, the concept of self-igniting small particles aligns perfectly with this definition. Pyrophoric materials are often finely divided solids that generate enough heat due to oxidation to ignite without an external ignition source. This characteristic can present significant handling and storage challenges, particularly in munitions systems, where safety protocols must be strictly followed to avoid inadvertent ignition.

The other choices do not accurately describe pyrophoric materials. Highly explosive pertains to substances that detonate, rather than those that ignite under certain conditions. Reacting to water is characteristic of water-reactive materials, which can undergo chemical changes when in contact with water. Stability under pressure refers to materials that do not break down, deform, or undergo chemical changes when subjected to pressure, which is a different concept entirely. Thus, the selected answer captures the specific and critical nature of pyrophoric substances.