H20

Hydrogen has the lowest ignition energy of any potential fuel gas. A spark of 0.2μJ is sufficient to set it off. Hydrogen combustion produces water, though at the temperatures normally encountered during combustion it is likely to be as steam, i.e. gaseous water.

At the risk of being overly detailed, you might also remind the questioner that oxygen is needed for hydrogen combustion, either from the separated oxygen, or from another source (air), and that the spark ignition of a hydrogen - oxygen mixture can be very potent! Hydrogen will also "burn" in fluorine, for example.

While you know the formula of water as H2O, its actual formula is H-OH, hydrogen hydroxide. Water tends to dissociate into hydrogen (H), which carries a positive charge, and hydroxyl (OH), which carries a negative charge, ions, by itself naturally. This is the basis for virtually all of the chemical reactions involving water (e.g. 2Na + 2H2O = 2NaOH + H2). Water can be broken down into hydrogen and oxygen through electrolysis. It can be recombined by burning and condensing the water vapor produced.

If you want to make a pollution free source of energy, electrolyze pure water to hydrogen and oxygen (using energy from nuclear power), then combust the two components back to water. This, of course, is not cheap.

And also inform the poster that a hydrogen flame produces only UV light, and is invisible to the human eye. Due to the lack of odor, any leak can become a flame source or an explosive environment without extreme care. Watch out for heat ripples in the air.

RichH

It takes energy to separate water into Hydrogen and Oxygen.

When they recombine, (which they will do entusiastically at the slightest provocation),

you get that energy back, very quickly.

Careful. It was curiousity killed the cat.