What phenomenon results from exothermic reactions occurring spontaneously in certain materials?

The phenomenon that results from exothermic reactions occurring spontaneously in certain materials is self-heating. This process involves a material generating heat through chemical reactions without an external heat source. Over time, if the heat produced exceeds the heat loss to the environment, the temperature of the material may rise significantly.

Self-heating is particularly concerning in materials like organic matter, such as hay or compost, or certain chemicals that can decompose or react even in the absence of ignition sources. The risk lies in the potential for this heat buildup to eventually lead to ignition, especially if the material is in a confined space where heat cannot escape.

In contrast, the other options describe different phenomena. Self-ignition refers to the point at which material reaches a temperature sufficient to ignite spontaneously, often after self-heating has increased the temperature beyond its ignition threshold. Smoke explosions involve a rapid combustion of smoke and fuel, which is a result of a mixture of combustible materials and an ignition source, rather than a spontaneous reaction. Smoldering is a slow, flameless form of combustion that often occurs at lower temperatures, typically following self-heating, but does not necessarily denote a spontaneous reaction.

Thus, self-heating is accurately the phenomenon characterized by spontaneous exothermic