Class D agents work primarily by...

• A. Forming a crust over the fuel
• B. Smothering with foam
• C. Cooling with water
• D. Absorbing heat

Answer: (A)

Class D extinguishing agents are designed for fires involving burning metals, and their main action is to form a crust or solid blanket over the hot metal surface. This barrier isolates the fuel from air and prevents ongoing oxidation, which is the chemical reaction keeping a metal fire going. By covering the molten metal, the crust also helps prevent splatter and re-ignition as the surface is no longer exposed to oxygen. Cooling with water or absorbing heat can help other types of fires, but with metal fires those methods are unreliable and can be dangerous—water can react violently with certain metals, and foams don’t effectively halt the oxidation at the metal surface. So, the crust that fences the metal from air is the key mechanism by which Class D extinguishing agents work.