Hydrogen gas ( '' ' dihydrogen ' '' or molecular hydrogen , also called '' ' diprotium ' '' when consisting specifically of a pair of protium atoms ) is highly flammable : The enthalpy of combustion is − 286 kJ/mol : Hydrogen gas forms explosive mixtures with air in concentrations from 4 – 74 % and with chlorine at 5 – 95 % . The explosive reactions may be triggered by spark , heat , or sunlight . The hydrogen autoignition temperature , the temperature of spontaneous ignition in air , is . Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye , as illustrated by the faint plume of the Space Shuttle Main Engine , compared to the highly visible plume of a Space Shuttle Solid Rocket Booster , which uses an ammonium perchlorate composite . The detection of a burning hydrogen leak may require a flame detector ; such leaks can be very dangerous . Hydrogen flames in other conditions are blue , resembling blue natural gas flames . The destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated . The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin
destruction of the Hindenburg airship
Hindenburg disaster
Hydrogen