Why is water used as a fire extinguisher?

One might wonder, Hydrogen is highly combustible gas and oxygen helps it in combustion. Then why does the combination of both of them as H2O in the form of water extinguish fire?

Here's the answer: There is a difference between mixture and compound. At the end of chemical combination the form of substance changes completely. Its properties become different from the properties of the constituent elements. The constituent elements of a compound can not be separated easily. Accompanying diagram gives an idea of how an atom of oxygen bounds strongly with two atoms of hydrogen. Each atom of oxygen has six electrons orbiting around it but still has room for two more electrons. If suitable electrons of some other substance are available then oxygen accepts them and forms a permanent bond. A hydrogen atom has one such electron. Since it has only one electron, two atoms of hydrogen fill that gap with their two electrons. This bounding of atoms of oxygen and hydrogen elements creates a chemical H2O, i.e. water having entirely different properties. Bounded atoms of oxygen and hydrogen no longer retain their original properties.

Now water having different properties is an extinguisher of fire. It breaks contact of the burning material with the air in the atmosphere. Combustions stop without oxygen of the air. However, there is one condition. If the burning material is a liquid then the weight of water should be less than that of burning liquid. Petrol is lighter than water so the fire of burning petrol can not be extinguished easily with water. Being heavier water settles below the layer of petrol which continues to remain in contact with the air as before.

If water is an exemplary fire extinguisher it is because no substance matches it in absorption of heat. It can absorb tremendous amount of heat. Assume that one pound each of gold, iron and water are completely devoid of heat. Temperature of each material is 273º Celsius below zero. Scientifically this temperature is considered as absolute zero because the substance does not have any heat at this point. Obviously water will be in the form of ice. On heating these substances in such a way that heat is evenly absorbed by them it is found that gold starts melting first at 1102º Celsius but at this stage temperature ice is no more than 149º Celsius below zero and when iron stars melting at 1299º Celsius the temperature of ice barely reaches 0º Celsius! Ice starts melting gradually taking the form of water only thereafter. And it will take still more time and heat before it starts boiling.

Consider some figures again: Ordinary paper catches fire at 184º Celsius. Cotton burns at 228º Celsius. Combustion point of wood is in the range of 240º to 269º Celsius. As the water poured by the fire engine easily absorbs heat of most of these burning objects, flames have got to be extinguished!

More reading:
Water (properties) (Wikipedia)
Water (Wikipedia)

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