Associate Safety Professional Practice Exam

Which dry chemical is most effective for extinguishing fires from ordinary combustibles?

1. Sodium bicarbonate

2. Ammonium phosphate

3. Potassium sulfate

4. Calcium carbonate

The correct answer is: Ammonium phosphate

Ammonium phosphate is particularly effective for extinguishing fires involving ordinary combustibles, which typically include materials like wood, paper, and cloth. It acts as a flame inhibitor by forming a barrier between the fuel and the heat source, effectively cooling the burning material and preventing reignition. Its versatility allows it to not only smother the flames but also to prevent further combustion by forming a charred layer over the burning surface. This characteristic makes ammonium phosphate a common choice in multipurpose dry chemical extinguishers, particularly the Class A extinguishers that are rated for fires involving ordinary combustibles. Other options might not provide the same effectiveness for this particular type of fire. For example, sodium bicarbonate and calcium carbonate are primarily used for Class B fires (flamable liquids) and have limitations in their application for solid combustibles. Potassium sulfate has specialized uses in fire extinguishing but is less common compared to ammonium phosphate for ordinary combustibles. Therefore, the effectiveness and applicability of ammonium phosphate in a broader range of fire scenarios make it the preferred choice in this context.