What happens when unburned gases combine with newly admitted oxygen in a backdraft situation?

In a backdraft situation, unburned gases that have accumulated in a compartment are typically at a high temperature and contain a significant amount of flammable materials. When newly admitted oxygen is introduced into this environment—often due to an opening created by firefighters or ventilation efforts—the combination of the oxygen with the unburned gases leads to a rapid and intense combustion reaction.

This reaction is characterized by an explosive nature, primarily because the mixture can reach the appropriate fuel-to-air ratio necessary for combustion. The explosion occurs due to the sudden ignition of these highly concentrated flammable gases, which can propel flames outward violently and pose a serious risk to anyone in the vicinity. Understanding this phenomenon is crucial for fire suppression tactics, as it highlights the need for caution when ventilating structures that may contain unburned combustible gases.

Thus, the correct answer reflects the violent and dangerous nature of the reaction that occurs when unburned gases mix with oxygen in a backdraft scenario.