Why Does Water Extinguish Fire?

The Fire

It is often said that fire needs three things to burn: heat, fuel and oxygen. Flammable materials such as wood or gasoline have stored energy. When molecules of the flammable material are heated to a certain point, these materials react with any nearby oxygen, becoming carbon dioxide, carbon monoxide dioxide ash and other molecules; releasing heat and light. That heat can make nearby molecules react with the oxygen, creating the chain reaction we know as fire. As long as there is more fuel, more oxygen to react to and enough heat to sustain the reaction, the fire will continue.

Smothering the Fire

When water is used to quench a fire, it is sprayed at the base of the fire which is the actual burning fuel. The water coats the fuel, creating a barrier. It prevents oxygen from reaching the fire. Without oxygen, the fire will completely smother and die. Of course, if the fire is hot enough, the water will evaporate before is completely quenches the fire. Fortunately, this evaporation also helps to quench the fire.

Evaporative Cooling

When the water hits the fire, it is at a relatively cool temperature, usually around room temperature or a bit lower. In order to boil the water, the fire has to heat it up to 212 degrees Fahrenheit. The fire has to pour a tremendous amount of heat energy into the water to make it boil. When the water does boil, it carries that heat away from the fire, cooling it. This slows down the fire and also helps to put it out. If the fire is cooled enough, it won't be able to sustain the reaction, and will go out.

References

• How Water Extinguishes Fire