Final answer:
Combustible materials can be difficult to remove when extinguishing fires due to their properties such as releasing flammable gases or having high ignition temperatures, and because some materials burn very hot and release a large amount of energy.
Step-by-step explanation:
The removal of combustible material is not always possible when extinguishing a fire due to several reasons. Some materials release flammable gases or have a high ignition temperature, making them difficult to extinguish. Furthermore, certain combustible materials, such as crude oil, burn very hot and release a significant amount of energy when burned, which adds to the difficulty of extinguishment. For instance, it has been calculated that each liter of crude oil releases 2.80×107 J of energy when burned. This makes fires involving such materials particularly challenging to deal with. Chemical energy is released as heat when these materials are ignited, and this reaction is often irreversible, leaving a permanent change in the environment.