Class D agents work primarily by...

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)

Metals that burn in Class D fires rely on a surface oxidation reaction. The extinguishing approach for these fires uses dry powders that coat the burning metal, forming a crust over the fuel. This crust creates a barrier between the metal and the air, substantially reducing or stopping the oxidation reaction and thereby controlling the fire. Some heat absorption occurs, but the crucial effect is this protective layer that isolates the fuel from oxygen. Foams or water aren’t suitable for metal fires—they can react violently with certain metals or fail to establish the necessary crust—while absorbing heat alone doesn’t interrupt the chemical reaction as effectively as the crust does.