Flour on fire

Flour explodes when its particles become suspended in the air in a dust cloud and are then ignited. The starch molecules burn relatively quickly, and it is their rapid expansion in the presence of heat that causes an explosion.

Flour is not prone to explode all on its own. The individual grains must be separated and exposed to oxygen for there to be any risk. When stored in densely packed bags or containers, the chances of fire are quite low. Explosion becomes likely only when individual particles are suspended in the air, usually in the form of a dust cloud. Dust clouds in confined spaces both allow the starch molecules ample access to oxygen, and prevent escape. Under these conditions, any heat or heat source can set the sugar molecules ablaze.