Which materials are classified as pyrophoric?

Pyrophoric materials are those that ignite spontaneously upon exposure to air, due to their highly reactive nature. In this context, lithium, magnesium, and cesium are correctly identified as pyrophoric substances.

Lithium is well-known for its reactivity, particularly when it comes into contact with moisture and air, which leads to rapid oxidation and produces enough heat for ignition. Magnesium, also a reactive metal, can ignite at temperatures lower than those required for many other metals, especially in fine powder form. Cesium, another alkali metal, is extremely reactive and will ignite spontaneously when exposed to air.

The combination of these three materials highlights a group that poses significant safety concerns in handling and storage due to their propensity to catch fire easily when not properly contained. In comparison, the other combinations listed do not include this specific trio of pyrophoric elements, demonstrating a lack of awareness or recognition of all the key materials that exhibit this property.