What process must products such as wood and paper undergo to produce char?

To produce char, products like wood and paper must undergo a process known as pyrolysis. Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere, which means that it occurs in the absence of oxygen. During this process, organic materials break down chemically, which leads to the formation of char, a carbon-rich residue.

In the context of fire investigation, understanding pyrolysis is essential because it helps fire investigators determine the fire's behavior and the materials involved. When materials such as wood and paper are subjected to pyrolysis, they lose volatile compounds and moisture, eventually resulting in the creation of char. This char can provide critical information regarding the fire dynamics and the conditions before the fire.

Other processes mentioned, like combustion, evaporation, and condensation, do not lead to the creation of char. Combustion involves burning in the presence of oxygen, which produces heat, light, water vapor, and carbon dioxide, but not char. Evaporation and condensation relate to phase changes in liquids and gases and are not relevant to the formation of char from solid materials. Thus, pyrolysis is the correct answer for producing char from products like wood and paper in a fire investigation context.