Published: 6 October, 2008

Hydrogen molecule turns into water molecule through oxidation

What happens to a hydrogen atom after it has come in contact with a flame - I know it "pops" but what happens to the actual atom - does it remain as a hydrogen atom? Does it form a new atom or compund or is it annihilated?

Hydrogen gas is actually a molecule made up of two atoms of hydrogen and thus what really comes into contact with the flame is not a hydrogen atom but a hydrogen molecule. Simply stated, when ignited in a flame the hydrogen gas "burns" which is what produces the "pop". That is, in this reaction, hydrogen combines with oxygen to form a new molecule, water, which contains two atoms of hydrogen and one atom of water. However, the actual combustion of hydrogen in air is quite complex with many mechanistic steps and reaction products that are beyond the scope of this article.

In order to get a "pop", there must be enough oxygen present to oxidize (burn) the hydrogen very rapidly. If there is not enough oxygen for an explosion, the hydrogen will just burn with a flame. You may have seen pictures of the hydrogen gas-filled German zeppelin Hindenburg that caught fire while landing in New Jersey in 1937. The films of this disaster clearly show that the hydrogen gas burned fiercely, but did not explode in the usual sense of the word. However, this event led to the replacement of hydrogen by safe, non-combustible helium in subsequent dirigibles.

Now back to your question. What you may have experienced when you heard a "pop" was the reaction of hydrogen with oxygen such as produced after the electrolysis of water. During electrolysis, electricity is used to break down (dissociate) water molecules into their constituent atoms, which, as noted above, are two atoms of hydrogen and one of oxygen. At this ratio, called a stoichiometric amount, hydrogen will indeed produce a "pop". If a larger quantity of this ratio of the gases is ignited, a real explosion could occur with damaging effects.

Thus, the hydrogen does form a new compound: water. The hydrogen atom is still a hydrogen atom, that doesn't change. But what changes is its transformation from a hydrogen molecule into a water molecule by the process of oxidation. Furthermore, it is not annihilated. Annihilation is a physics term used for the process whereby matter and antimatter interact in which both disappear by conversion to energy. Thus, annihilation is a much more energetic reaction than the burning of hydrogen.