Substances that ignite when exposed to air at normal temperatures are ___________.

• A. Combustible
• B. Pyrophoric
• C. Oxidizing
• D. Spontaneous

Answer: (B)

Pyrophoric materials ignite upon contact with air at normal room temperatures because they react extremely quickly with oxygen (and often moisture) in the atmosphere. That rapid oxidation releases enough heat to start a fire without an external ignition source. This distinguishes them from combustibles, which need an ignition source to start burning; from oxidizers, which provide oxygen but don’t necessarily ignite on their own; and from spontaneous (self-heating) materials, which may ignite after a period of internal heat buildup rather than immediately upon air exposure. So the best term for substances that ignite just by being exposed to air at room temperature is pyrophoric.