Which condition generally promotes spontaneous combustion?

Spontaneous combustion occurs when a material reaches its ignition temperature without an external flame or spark, often due to the buildup of heat from chemical reactions, particularly oxidation. In various substances, such as oily rags or compost heaps, oxidation can produce heat as the chemical bonds in the material are broken down. If this heat is not dissipated—due to the material being tightly packed or insulated—it can increase the temperature to a point where ignition occurs.

This reaction is particularly relevant in scenarios involving organic materials that can oxidize and generate heat. As the process continues, it can lead to an uncontrollable increase in temperature, resulting in a fire. Understanding this concept is crucial for safely handling materials that are prone to spontaneous combustion, as effective measures such as proper storage and ventilation can mitigate the associated risks.

Other conditions mentioned, such as excessive cooling, the presence of a nearby flame, or isolation from air sources, do not contribute to spontaneous combustion in the same manner. Cooling would typically reduce the likelihood of combustion, while a flame would initiate combustion rather than allow for it to occur spontaneously. Isolation may prevent oxygen from reaching a material but does not enhance the heat buildup necessary for spontaneous ignition.