How does a fire extinguisher work?
In order for something to burn, high heat and oxygen are needed. All fuels have their own particular temperatures at which they begin to burn when exposed to high heat (called their flash points). Removing heat or oxygen from fuel will put out a fire. Water is frequently used to extinguish fires. Large supplies of water can be found almost anywhere, an important condition when dealing with large fires, like those in burning buildings. Water works in two ways to put out a fire. First, it sharply reduces the temperature of the burning material. Second, it covers the material, keeping oxygen-filled air from reaching the material. But water cannot put out oil fires. Because oil floats on the surface of water, an oil fire’s oxygen supply can’t be cut off by water. Other substances—liquids, gases, or powders that don’t burn—must be used to smother the fire and remove its oxygen supply. Most fire extinguishers are filled with carbon dioxide, a heavy gas that prevents burning. When released, the gas forms a type of snowy foam that both covers and cools a fire. Powdered sodium bicarbonate (what we know as baking soda) is also used in extinguishers, usually for use on oily chemical fires. It quickly melts in heat, forming a crust that keeps oxygen out. (If you do not have a fire extinguisher on hand you should always throw baking soda on a cooking fire that involves grease; water will only spread the fire by causing splattering.) Because the substance in a fire extinguisher must cover a large area very quickly, it needs to be released in a powerful spray. The extinguishing substance is stored inside the tank under high pressure, which drives it out of a nozzle with great force once it is released.