Which statement best defines a pyrophoric material?

Which statement best defines a pyrophoric material?

• A. A liquid or solid that when in contact with air and without an external ignition source can be ignited within 5 minutes.
• B. A material that only ignites with a continuous external heat source.
• C. A substance that cannot ignite in air.
• D. A gas that explodes upon abrupt release of pressure.

Answer: (A)

Pyrophoric materials ignite spontaneously when exposed to air due to a rapid exothermic reaction with oxygen. The statement that best fits this behavior describes a liquid or solid that, once in contact with air and without an external ignition source, can be ignited within a short time. This captures the defining trait: no external flame or heat is needed—the material can ignite simply from its reaction with air. In practice, these substances pose an ignition risk as soon as they are exposed to air, even without an intentional ignition source. If a material were to require a continuous external heat source to ignite, or if it cannot ignite in air at all, it would not be pyrophoric. Likewise, a gas that explodes on sudden pressure release describes a different hazard altogether, not spontaneous ignition in air.

Pyrophoric materials ignite spontaneously when exposed to air due to a rapid exothermic reaction with oxygen. The statement that best fits this behavior describes a liquid or solid that, once in contact with air and without an external ignition source, can be ignited within a short time. This captures the defining trait: no external flame or heat is needed—the material can ignite simply from its reaction with air.

In practice, these substances pose an ignition risk as soon as they are exposed to air, even without an intentional ignition source. If a material were to require a continuous external heat source to ignite, or if it cannot ignite in air at all, it would not be pyrophoric. Likewise, a gas that explodes on sudden pressure release describes a different hazard altogether, not spontaneous ignition in air.