What type of reaction involves heat and mass transfer through the unreacted medium?

Deflagration is a type of combustion reaction characterized by a subsonic propagation of the combustion wave through the unreacted medium. In this process, heat and mass transfer occur as the reaction front moves slowly compared to the speed of sound. The heat generated by the reaction raises the temperature of adjacent unreacted material, causing it to ignite progressively, rather than instantaneously. This gradual transfer of heat and mass is crucial, as it allows for a controlled burning process, often seen in common explosive materials and fuels.

In contrast, detonation refers to a supersonic combustion wave that propagates through an explosive material at a much faster rate, generating a shock wave rather than allowing for the gradual heat transfer that characterizes deflagration. Thermal explosions typically occur when a material self-heats and eventually ignites, usually in an enclosed space, without the initial involvement of an external ignition source. Simultaneous reactions are not a recognized category of combustion in the context of heat and mass transfer dynamics as defined in explosion hazards. Thus, deflagration is the best choice as it embodies the crucial aspects of heat and mass transfer through the unreacted medium during the combustion process.