Readily combustible solids ignite by which mechanisms?

Readily combustible solids ignite by which mechanisms?

• A. Solids that do not ignite.
• B. Solids that burn only when heated.
• C. Solids that ignite spontaneously without any friction.
• D. Solids that burn through friction, burn faster than 2.2 mm per second, or any metal powder that can be ignited.

Answer: (D)

Readily combustible solids are those that can catch fire without a flame when prompted by certain initiating conditions. The primary ways this can happen are frictional heating, spontaneous heating, and ignition of finely divided metal powders. Friction from handling, grinding, or striking can generate enough heat locally to reach ignition temperatures. Some solids heat up by themselves as they oxidize, and if this self-heating occurs rapidly enough, ignition can happen without an external heat source. Finely divided metals have a large surface area and burn readily when dispersed in air, making metal powders particularly easy to ignite. Standards often use a practical threshold, such as a burning rate faster than 2.2 mm per second, to identify materials that ignite readily. These mechanisms—friction, spontaneous heating, and metal powder ignition—cover the recognized ways readily combustible solids can ignite, so this option is the best fit.

Readily combustible solids are those that can catch fire without a flame when prompted by certain initiating conditions. The primary ways this can happen are frictional heating, spontaneous heating, and ignition of finely divided metal powders. Friction from handling, grinding, or striking can generate enough heat locally to reach ignition temperatures. Some solids heat up by themselves as they oxidize, and if this self-heating occurs rapidly enough, ignition can happen without an external heat source. Finely divided metals have a large surface area and burn readily when dispersed in air, making metal powders particularly easy to ignite. Standards often use a practical threshold, such as a burning rate faster than 2.2 mm per second, to identify materials that ignite readily. These mechanisms—friction, spontaneous heating, and metal powder ignition—cover the recognized ways readily combustible solids can ignite, so this option is the best fit.