As children, we were told that water puts out fire. This aspect of fire safety is very important, but we do not usually learn the science of how this liquid subdues flames.
How Does Water Put Out Fire?
The ability of water to put out fire might be different than most people think. According to fire combustion and behavior expert Sara McAllister from the U.S. Forest Service's Missoula Fire Sciences Laboratory, water extinguishes fire mainly because it is a perfect heat sink. In other words, this liquid is good at absorbing heat.
Water does not act on the flames themselves. For a fire to keep going, it requires three things: fuel, oxygen, and a heat source. Instead of fighting the fire directly, water acts as the fuel, making it hard to keep burning.
When wood is burning, for example, the heat of the fire vaporizes substances in the wood, turning them into gas, which fuels the fire. If the wood is doused with water, the flame must not be hot enough to vaporize both the water and the wood.
Because water has a high heat capacity, vaporizing it will take a lot of energy. If the fire spends energy trying to vaporize the water, it has less energy to heat the fuel. As heat is absorbed by water, the fuel is cooled. Without heating enough fuel to vaporize the water, the flame cannot feed itself, dissipating.
This ability of water is a very effective way to contain a fire, where the potential fuel is made wet to prevent the fire from spreading. This same strategy is the one used in sprinkler systems. Since they prevent the fire from spreading further, firefighters can have enough time to get to the scene.
Vaporized water can also be a good fire-extinguishing strategy for special scenarios. For example, pouring water on all the computer units is not ideal in a computer room. In enclosed spaces, a strategy known as misting can help preserve the equipment. In this approach, the tiny droplets act like vapor, which is misted into a room. Fire responders can get so much water vapor in the area that it displaces the oxygen and cools the flame.
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When Is Water Not an Effective Fire Extinguisher?
Water is an effective extinguishing agent, but there are also situations where water is not a feasible approach. Fires are categorized into five classes, each involving a different flammable material.
Class A fires are caused by solid materials such as paper, plastic, clothing, and wood. Class B fires involve flammable liquids like gasoline, alcohol, and oil. Class C fires are brought by electricity, and Class D fires involve metal ignition. Meanwhile, Class K fires are caused by cooking liquids and fats and may sometimes be grouped with Class B.
Generally, water can only be used to extinguish Class A fires. On the other hand, using water in putting out Class B and Class B fires is not recommended.
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