How Fire Extinguishers Work

Q: When is a water extinguisher dangerous?
A: A water extinguisher can put out things like burning wood, paper or cardboard, but it doesn't work well on electrical fires or fires involving inflammable liquids. In an electrical fire, the water may conduct the current, which can electrocute you.

A fire extinguisher is an absolute necessity in any home or office. While there's a good chance that the extinguisher will sit on the wall for years, collecting dust, it could end up saving your property and even your life.

In this article, we'll see exactly what fire extinguishers do and how they do it. We'll also find out what causes fire in the first place, learn the correct way to use an extinguisher and see what sort of fire suppressant works best on different types of fires.

What is Fire?
Extinguishing a Fire
We don't usually think much about how a fire extinguisher works -- until we need to use one.
Fire is the result of a chemical combustion reaction, typically a reaction between oxygen in the atmosphere and some sort of fuel (wood or gasoline, for example). Of course, wood and gasoline don't spontaneously catch on fire just because they're surrounded by oxygen. For the combustion reaction to take place, the fuel has to be heated to its ignition temperature.­

­Here's the sequence of events in a typical wood fire:
- Something heats the wood to very high temperatures. This could be any number of things -- focused light, friction, something else that is already burning.
- When the wood reaches about 500 degrees Fahrenheit (260 degrees Celsius), the heat decomposes some of the cellulose material that makes up the wood.
- Decomposed material is released as volatile gases, typically a compound of hydrogen, carbon and oxygen.
- When the gas is hot enough, the compound molecules break apart, and the atoms recombine with the oxygen to form water, carbon dioxide and other products.
- The gases, which rise through the air, make up the flame. Carbon atoms rising in the flame emit light as they heat up. (Check out How Light Bulbs Work to find out why heated objects emit light.)
- The heat of the flame keeps the fuel at the ignition temperature, so it continues to burn as long as there is fuel and oxygen.

As you can see, there are three essential elements involved in this process:
- Extreme heat
- Oxygen (or similar gas)
- Fuel

Fire extinguishers are designed to remove at least one of these elements so that a fire will die out. There are several different ways of doing this, as we'll see in the next section.

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