What is defined as the lowest temperature at which a substance ignites spontaneously?

The correct answer is auto ignition temperature, which refers to the lowest temperature at which a substance will ignite and sustain combustion without an external ignition source, such as a spark or flame. At this specific temperature, the heat generated by the material itself through mechanisms like compression or reaction with atmospheric oxygen is sufficient to initiate combustion.

Understanding this concept is crucial in fire safety and prevention efforts, as materials may not require a direct source of ignition to catch fire; rather, they can self-ignite when exposed to sufficient heat for a prolonged period. The auto ignition temperature is a key parameter in assessing the flammability and risk of materials in various environments.

In contrast, the other terms provided describe different ignition characteristics. The flash point is the lowest temperature at which the vapor of a flammable substance can ignite in the presence of an ignition source, but it does not indicate spontaneous combustion without such a source. The flame point, while closely related, refers specifically to the temperature at which a flame is produced upon ignition of a vapor-air mixture. Spontaneous ignition temperature is often used interchangeably with auto ignition temperature, but it may not be as widely recognized in technical definitions as the latter. Overall, auto ignition temperature is the most precise term describing the phenomenon of spontaneous