Why is oxygen flammable?
Oxygen is available in the atmosphere and is utilized by the animals for their survival. Generally fuels are flammable like petrol, diesel, kerosene, cooking gas and so on. But oxygen is not a fuel to have this property of inflammability. The flame or fire is initiated and developed when the fuel is oxidized or combined with oxygen. Applying oxygen to a burning fuel will enhance the burning process and increases the size of the flame. The fire will be increased vigorously when oxygen is added to it.
Oxygen by itself is not flammable. If that is the case, 21 percent of atmosphere that is made up of oxygen might have got burnt by now. The fuels if mixed with oxygen burn faster than alone by themselves. The cutting torches that are used for cutting the metal are mixed with oxygen to result in a very hot flame which can cut the metal very easily and quickly. In the creation of a rocket, NASA uses oxygen to burn the hydrogen fuel so that hydrogen can burn faster in the engine. The oxygen supplementation is done in the rocket as there will be reduction of oxygen levels as it goes higher and higher into the space.
Any oxygen rich sources would cause rapid combustion. When concentrated oxidants and fuel are brought closer, fire or rapid explosion might result. A small fire or spark is enough to trigger the big fire. The oxygen rich compounds have high oxidative potential and are able to cause initiation of combustion. Some of these compounds are peroxides, chlorates, nitrates, per chlorates, and di chromates which can readily provide oxygen to the fire. Any oxygen system has to be designed and manufactured carefully that no ignition sources are attached to it. The Apollo 1 was crew was killed in the fire that got expanded so rapidly in the launch pad as the capsule was filled with pure oxygen. The gas was filled with the pressure higher than the atmospheric pressure.