What immediate chemical process begins when sufficient heat is present in a fuel-oxygen system?

• A. Free radicals form and initiate a self-sustained chemical reaction.
• B. Oxygen is consumed and stops the reaction.
• C. Vaporization occurs without radical formation.
• D. Moisture reacts to form inert gases.

Answer: (A)

Ignition begins when enough heat creates free radicals that initiate a self-sustaining chain reaction. In a fuel-oxygen mixture, the thermal energy overcomes activation barriers and splits molecules to form reactive fragments like H, O, and OH. These radicals rapidly react with fuel and oxygen, producing more radicals and heat, which drives the rapid, continuous combustion that keeps the flame going. This initiation phase is what makes the reaction self-sustaining once it starts. Vaporization alone is a physical change that doesn’t by itself start a chemical chain reaction, moisture forming inert gases would tend to dampen or dilute the reactants, and oxygen being consumed without radical-driven propagation wouldn’t explain a self-sustaining flame.