Under what conditions can hydrogen and oxygen ignite or explode?

Mixtures of hydrogen and oxygen can ignite or explode under specific conditions involving temperature, concentration, and external triggers.

• Temperature: While hydrogen and oxygen react at a measurable rate only above 300° C, mixtures containing between 4 and 94 percent hydrogen will ignite when heated to temperatures between 550° C and 600° C.1
• External Triggers: Even if the temperature is below the auto-ignition range, these mixtures can be ignited by a spark, a flame, or contact with a catalyst.1
• Concentration: The explosive potential varies with the ratio of the gases. A 2:1 mixture of hydrogen and oxygen is particularly volatile, resulting in an especially violent explosion.1

The combustion process itself is a complex, branched-chain reaction that produces water as the final product. Factors such as explosion limits and ignition delays are determined by the interaction of hydrogen and oxygen atoms and molecules, which can be quantitatively described through empirical rate constants.2