Why is the only byproduct of burning hydrogen water?
The word "hydrogen" is Greek for "water-former".
If fuels are burned, the elements in the fuel combine with oxygen to form oxides. In hydrogen only hydrogen-oxide (=water) can be formed, as no other elements are present.
Most fuels contain other elements, like carbon (
Carbon combines with oxygen to carbondioxide (
Example : natural gas or methane (
So the carbon-atom burns to carbondioxide, and the hydrogen burns to water (vapour).