Fire

What is fire? Fire is the product of a chemical reaction. When we see fire, we are seeing very hot gasses. In most fires, these gasses are carbon dioxide and steam. The gasses are heated by energy released by the chemical reaction.

How do you put out a fire? Throw water on it? That will work for many fires, but not all. It will even make some fires worse. To know how to put out a fire safely, you need to know more about fires.

Fire Every fire needs three things. Oxygen to combine with the fuel. Fuel to provide energy. And heat to unlock the energy in the fuel. Take one away and the fire dies.

Oxygen Most of the oxygen in the air we breath is diatomic (two oxygen atoms stuck together—O 2 ). This is fairly stable at low temperatures, but when heated it becomes unstable, ready to react with many different substances.

Fuel A fuel is a substance that can react with oxygen to form a more stable compound. Because the product of the reaction is more stable than the oxygen and fuel by themselves, the reaction releases energy in the form of heat.

Heat Oxygen and fuel are stable at normal temperatures. Heat must be added to start the reaction. This is sometimes called the “ignition energy”. Because burning fuels release more heat than is needed to ignite them, a chain reaction results. Each fuel molecule that combines with oxygen supplies the energy needed for the next reaction.

What does all that have to do with putting out a fire? Think of a fire as a chair with three legs. If you take away one of the legs, what happens to the chair? Remove the fuel, and there is no source of energy. Remove the oxygen and the fuel has nothing to react with. Remove the heat and both the oxygen and the fuel become stable and unreactive.

Fight fire with water Water is good at putting out many fires because it attacks two sides of the fire triangle. Water has a high specific heat capacity and a very high latent heat of vaporization—water thrown onto a fire boils quickly, but as it does it absorbs a lot of heat. This lowers the temperature and makes the fuel and oxygen more stable. Water and steam also create a barrier, separating the fuel from the oxygen. If fuel and oxygen can’t get together, there can be no reaction.

When not to use water Many fires are cause by faulty electrics. Short circuits and poor grounding can lead to sparks and overheating. While water would work well to put out the fire, it can also cause more short circuits and can even electrocute the person throwing the water.

When not to use water Water is more dense than most liquid fuels, water thrown on them will settle to the bottom and fail to smother the fire—it may even cause the fuel to overflow its container. The hot fuel can also heat the water to boiling, if this happens quickly, the rush of steam can throw fuel into the air allowing to combine more easily with oxygen.

When you should use water Water is best for putting out solid fuels. Solid fuels tend to be more dense than water so they aren’t scattered. This also allows the water to cover them cutting off oxygen.

What to use instead of water Fire extinguishers come in three main types— water, carbon dioxide, and chemical. Carbon dioxide extinguishers are good for electrical fires. They smother the flames and cool the temperature, without being conductive.

What to use instead of water Chemical extinguishers spray an a powder or foam that smothers the flames. They are less effective than water or CO 2 because they don’t cool the fire. But they do not risk spreading the fuel around.

What if I don’t have a fire extinguisher? Grease or oil fires are common in the kitchen. If you do not have a CO 2 or chemical extinguisher, there are things you can use. If you have a lid that will cover a burning pan, it can cut off the supply of oxygen. (Let the pan cool before removing the lid or the fire will start again.) Baking soda can also be spread across a fire to smother it.