If oxygen feeds fire and hydrogen is explosive, then why isn't water combustible?
The properties of individual atoms change completely when they combine into molecules. Every time you salt your food, for example, each molecule of salt contains one atom of chlorine, a poisonous gas, and one atom of sodium, a substance that explodes on contact with water!
Each element's chemical properties are determined mainly by its outermost electrons. The formation of molecules involves those same electrons, disrupting their pattern and the chemical properties they created for their 'naked' atoms.
Paul Walorski, B.A., Part Time Physics Instructor
Combustion usually has three requirements: 1)fuel, 2)oxidizer and 3)heat or source of ignition. In the case of hydrogen and oxygen, hydrogen is the fuel and oxygen is the oxidizer. Even after they are mixed, they still require a spark or other source heat to ignite the extremely rapid combustion that results in an explosion.
The process of burning in this case is the oxidation of hydrogen or combination of the hydrogen and oxygen molecules. The result of the oxidation of hydrogen, its combustion product, is simply H2O or water. The reason that water (in any state) is not flammable is that it is already the product of combustion.
The chemical equation for this reaction is:
2H2 + O2 -> 2H2O.
The reaction also releases a large amount of energy, mostly in the form of heat that causes the water vapor to expand rapidly to a large volume. This rapid expansion of gases is what causes the explosion. In fact, this is the basic principle of all explosives.
Scott Wilber, President, ComScire - Quantum World Corporation
'Our job in physics is to see things simply, to understand a great many complicated phenomena, in terms of a few simple principles.'