What is the primary function of the surfactants in Class A foam?

• A. They increase the surface tension of water.
• B. They evaporate quickly to provide cooling.
• C. They chemically neutralize the fuel.
• D. They reduce the surface tension of water in the foam solution, allowing better penetration into the fuel.

Answer: (D)

Surfactants in Class A foam primarily lower the surface tension of the water in the foam solution. This wetting action lets the foam spread more easily over fuel surfaces and penetrate into porous fuels and fuel–air interfaces, creating a cohesive foam blanket that improves cooling and fuel knockdown. They do not increase surface tension, evaporate quickly for cooling, or chemically neutralize the fuel.