The primary action of foam on a flammable-liquid fire is to

• A. Dilute the fuel
• B. Separate the fuel from the fire
• C. Extinguish the flame by cooling
• D. Create a vapor cloud

Answer: (B)

Foam's primary action on a flammable-liquid fire is to separate the fuel from the air by forming a stable foam blanket that floats on the liquid surface. This barrier keeps fuel vapors away from the flame and reduces the rate at which vapors are released, disrupting the fuel–air mix needed for sustained combustion. The cooling and smothering effects come from the water content in the foam and the blanket it creates, but they are secondary to the main mechanism of keeping fuel vapors away from the flame. It doesn’t dilute the fuel, and forming a vapor cloud would not help extinguish the fire.