What hydrogen releases upon burning depends on its environment and the type of burning it goes through. There are generally two ways hydrogen can burn: It can be used in nuclear fusion, in powerful reactions such as the ones that cause stars to burn, or it can combust on earth with the help of the oxygen-rich atmosphere. On earth, hydrogen can be found in many different substances, but pure hydrogen acts a particular way and only emits certain particles when burned.
Hydrogen is considered the most common chemical element in existence and is responsible for a large amount of the heat that exists in the universe. In nuclear reactions, especially those that power the sun and other stars, hydrogen is put under tremendous pressure until it releases a large amount of heat and light; it then reforms into other elements. The nuclear reaction uses up the hydrogen atom and fuses the leftover parts of several hydrogen atoms into a helium atom. This process actually changes depending on the size of the star, but helium is still the primary element produced. Other particles are also produced in smaller amounts, not unlike ashes left over from the nuclear fusion; these particles may eventually come together and create a neutron star after all the hydrogen and helium are gone.
Hydrogen as a Fuel
On earth, hydrogen does not go through the nuclear-reaction process at all unless forced to inside an atomic bomb. Instead, the atoms burn in a completely different way, similar to how hydrocarbon fuels burn, but in a purer form. Like the carbon-based fuels, pure hydrogen reacts with the air around it to combust and produces a large amount of heat as energy. Unlike the more common fuels, pure hydrogen does not leave behind many extra or contaminant particles.
The most common substance resulting from hydrogen combustion is water. The hydrogen atoms mix with the oxygen atoms and create the essential H20 formula, resulting in a light residue of water that may escape as water vapor or condense on surfaces near where the hydrogen is burned. Of course, air is only partly oxygen and there are other elements in the atmosphere, most notably nitrogen. When hydrogen burns, it also burns the nitrogen, and can release various oxides of nitrogen into the air.
Oxides of nitrogen are dangerous particles that can help create acid rain and take part in other destructive cycles. But pure hydrogen is still heralded as a clean fuel, primarily because the amount of oxides it creates is minimal compared to that of fossil fuels, and hydrogen's main by-product, water, is harmless. The most difficult steps in tapping hydrogen as a fuel are finding it in its pure form and using the energy it produces efficiently. Many scientific processes are employed to extract pure hydrogen from the various substances it has bonded with on earth.