Sparks or certain radiations can cause a mixture of hydrogen and chlorine to react explosively to yield hydrogen chloride, as represented by the equation H2 + Cl2 → 2HCl. Mixtures of hydrogen and oxygen react at a measurable rate only above 300° C, according to the equation 2H2 + O2 → 2H2O.
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In layman's terms, burning hydrogen results in water: H2 + 1 2 O2 −→ H2O + 286, 000 joules. This combustion reaction also releases 286,000 joules of energy per mole of hydrogen gas burned.