Which compound is added to a water extinguisher to improve penetration into fuel?

A wetting agent is a compound that is added to water extinguishers to enhance their effectiveness in suppressing fires, particularly those involving combustible materials like wood, paper, and some plastics. These agents have surfactant properties, which reduce the surface tension of water. This reduction allows the water to penetrate more effectively into fuels, making it easier for the water to absorb heat and cool down the flames.

By improving penetration, wetting agents ensure that the water can reach the burning material more efficiently, facilitating faster extinguishment of the fire. This is especially important in wildland firefighting, as well as in structure fires where deep-seated embers or hidden fires can be challenging to suppress.

The other compounds listed do not serve the same purpose as wetting agents. While they may have their uses in different contexts, they do not specifically improve water penetration into fuel in the way a wetting agent does.