# If aliens lived in a hydrogen (or any flammable gas) based atmosphere, would they perceive oxygen to be flammable?

Energy, fuel, and oxygen are needed for a fire to burn. In a hydrogen chamber, if there was a pipe spewing small amounts of oxygen, could you light the oxygen on fire as it began to mix with the hydrogen?

• Absolutely. Lighting a torch in such an environment would simply be the reverse physical process (and same chemical process) of what is done in our oxidizing atmosphere. In the chamber, or alien world of hydrogen gas, providing an ignition source to a stream of oxygen would give you a flame. The chemical reaction would actually be the same as if you had lit a hydrogen stream in an oxygen-containing atmosphere. Also, welcome to Chemistry.SE! – airhuff Apr 17 '17 at 0:33
• @Howzieky nobody said only — I wonder if that could be an interesting next question. – uhoh Apr 17 '17 at 0:45
• Yes, that is opposite from the way we're accustom to looking at things and it would seem strange. However, there are other oxidizing compounds besides oxygen that would react with hydrogen to form a flame. For example see this video of hydrogen burning with chlorine gas, where chlorine plays the role of oxygen as the oxidant! Note that the only purpose of the acetylene was to light the chlorine-hydrogen flame. – airhuff Apr 17 '17 at 0:48
• I'm really glad I said "only", otherwise I wouldn't have gotten the extra info! I never knew anything but oxygen could serve as an oxidant. How is that possible? – Howzieky Apr 17 '17 at 1:19
• Note also that an ignition source isn't required for all reactions. Some reactions will start on their own with no activation energy required. For example, sodium with water (don't try this at home) or Francium with water (don't try this anywhere.) – reirab Apr 17 '17 at 4:11

$$\ce{2H2 + O2 -> 2H2O + energy}$$
The only thing we've done is reversed the sources of the $\ce{H2}$ and $\ce{O2}$, so the hydrogen-breathing aliens would perceive oxygen to be explosively flammable, just as we perceive hydrogen gas to be so. The bottom line is that it is the two of the gases together that are flammable.