How Do Powder Fire Extinguishers Work. A dry chemical extinguisher sprays a very fine powder of sodium bicarbonate (nahco3, baking soda), potassium bicarbonate (khco3, nearly identical to baking soda), or monoammonium phosphate ( (nh4)h2po4). They spray out the powder and the powder spreads quickly and cover all burned area, thus put fire out.
When you put a layer of powder or foam on the fire, you cut the fuel off from the oxygen around it, and the fire goes out. Hence the fire is suppressed. Abc fire extinguishers use a dry chemical powder called monoammonium phosphate to put out the abc classes of fire.