After rapid knockdown with dry chemical, what should be applied to prevent flashback?

• A. Foam
• B. Water
• C. Dry chemical
• D. Halon

Answer: (A)

Cooling and fog-blanketing the fuel surface are the key ideas here. After you’ve knocked down the flame with dry chemical, lingering hydrocarbon vapors can still flash back if air can reach the fuel. Foam provides both cooling and a stable blanket on the spill, which seals off the air–fuel interface and reduces vapor formation. That blanket—and the cooling effect of the foam–water solution—helps prevent reignition as the area is stabilized and cooled. Water alone cools but doesn’t form the same vapor-suppressing barrier and can spread hydrocarbon fuels or fail to stop re-ignition once the surface reheats. Dry chemical by itself stops the flame but doesn’t give ongoing vapor suppression. Halon is not used for this purpose in modern practice due to environmental and availability considerations.