Why do fire extinguishers stop flames?
Fire extinguishers stop flames either by dousing them in water or by excluding the oxygen which a fire needs in order to burn.
There are three main kinds of fires. First are those occurring on ordinary materials like paper and wood for which the quenching and cooling effects of water or water solutions are the most effective. Second come those involving inflammable liquids or greases for which a blanketing or smothering effect is essential. Finally there are the fires occurring in “live” electrical equipment where a special extinguishing agent must be used.
The most common extinguisher for the first type of fire is a bucket of water, or a manufactured extinguisher with water containing a chemical. The chemical reaction expels the water which puts out the fire.
For the second kind of fire the most common method is to use a chemical extinguisher to spray the burning material with foam which puts out fire by excluding oxygen. This foam usually consist of bubbles of carbon dioxide. As the foam is not a conductor of electricity, it may also be used safely on the third type of fire.
Foam-type extinguishers can generally be employed safely in nearly all cases, but water or water solutions should never be used on oil, grease or electrical fire.
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