Materials such as lithium, finely divided magnesium, sodium, and cesium are classified as which type of reactive materials?

• A. Hypergolic
• B. Pyrophoric
• C. Water reactive
• D. Inhibitors

Answer: (C)

What this question tests is how certain metals react when they come into contact with water. Metals like lithium, sodium, cesium, and finely divided magnesium don’t just sit there; they react vigorously with water, producing hydrogen gas and a metal hydroxide plus a lot of heat. For example, when these metals meet water, reactions like Li + H2O → LiOH + H2 or Na + H2O → NaOH + H2 release hydrogen gas that can ignite, sometimes with the heat driving a flame or explosion. This behavior is the defining hazard captured by labeling them as water reactive.

The other options describe different hazards. Hyperbolic or hypergolic materials ignite on contact with certain oxidizers, which isn’t the specific issue here. Inhibitors are substances that slow reactions, not reactive materials. Pyrophoric means materials that ignite spontaneously in air, which can be true for some finely divided metals, but the classically emphasized hazard for these particular items in many safety classifications is their violent reaction with water, hence water reactive.