For the fire tetrahedron, which component is added that unites the three originally found in the fire triangle?

The correct answer is the chemical chain reaction. The fire tetrahedron expands upon the traditional fire triangle, which consists of heat, fuel, and oxygen. By adding the component of the chemical chain reaction, the tetrahedron accounts for the process that contributes to the ongoing combustion reaction.

In fire dynamics, the chemical chain reaction refers to the series of reactions that occur once a fire starts, which sustain the fire. This includes the breakdown of fuel molecules and the continuous release of energy and heat, driving the reaction forward. Without this chain reaction, once the initial energy source (heat) is applied to the fuel and oxygen, the fire would not sustain itself.

Oxygen, fuel, and heat are already part of the fire triangle and do not account for the necessary reactions taking place during combustion. Thus, incorporating the chemical chain reaction into the understanding of fire behavior helps fire service professionals better grasp how fires start, sustain, and can be extinguished.