Question 52 ONC05 - Master or Mate of LT 200 GRT

The Correct Answer is A. **Explanation for A (Dry chemical agents do little or no cooling):** Dry chemical agents primarily extinguish Class B fires (flammable liquids) by interrupting the chemical chain reaction of combustion (a process known as chemical flame inhibition). While this effectively puts the fire out, the agent does very little to reduce the actual temperature of the flammable liquid or its container. If the liquid remains hot—above its autoignition temperature or flash point—and is re-exposed to an ignition source (like a hot surface or static charge) before it cools down, or if the vapor blanket is disturbed, the fire can easily reignite, which is known as reflash. **Why other options are incorrect:** * **B) Dry chemical is not an effective agent on Class B fires:** This is incorrect. Dry chemical (especially multipurpose ABC and standard BC types) is the most common and generally highly effective agent for rapidly extinguishing Class B fires. * **C) It is rapidly absorbed by the liquid:** This is incorrect. Dry chemicals are fine powders (salts) that form a layer on the surface of the burning fuel or are suspended in the air; they are not soluble in or absorbed by the flammable liquids they are used on. * **D) Dry chemical dissipates quickly:** While the visible cloud of powder may disperse, the effectiveness of dry chemical is primarily through chemical interference in the flame zone, and the powder layer on the surface can help temporarily inhibit vaporization. However, the risk of reflash is fundamentally linked to the lack of cooling, not the speed at which the powder clears from the air.