A lump of coal burns at a moderate rate in air while coal dust burns explosively.why?
The coal dust burns explosively because of the greater surface area possessed by it so that more of the carbon (coal) exposed to the fire and hence it burns quicker and more easily than the lump of coal. Because of more surface area in coal dust all the particles are uniformly heated and thus they ignite instantly and thus may burn with an explosion. But in case of coal lump, the surface area to volume ratio is very low. The heating is not uniform because there are more particles inside that lump which can be reacted but because they are not exposed to air they do not directly burn thus it takes a longer time period for entire coal to react. Thus heating in this case is moderate.