# Can hydrogen and oxygen gasses be turned back into water?

I electrolized water today, and got hydrogen. And I have a bottle of oxygen. Can I combine those to get myself water? I know $\ce{H2O}$ is super pure water, but I just want to try. Is this too idiotic to ask or can it be done?

• Yes. If you heat up hydrogen in oxygen, the hydrogen will undergo combustion to give you water: $\ce{2H2 + O2 -> 2H2O}$
– DHMO
Commented Oct 3, 2016 at 11:15
• You don't need to heat it up. You just need a spark. Don't do it at home unless you have done proper research, this reaction is very exothermic and can potentially be dangerous. Commented Oct 3, 2016 at 11:18
• I tried sparking it but the Hydrogen just exploded.i guess the amount H/O was wrong.And i also tried it with a neon transformer and made plasma but it just somehow all disappeared.I trued both in and without vacuum, nothing worked sadly. Commented Oct 3, 2016 at 13:48
• You can run the hydrogen gas through a platinum sponge exposed to air - the hydrogen will burn instead of exploding (if done properly). Or use a hydrogen fuel cell for the most 'peaceful' reaction (electricity will be a nice bonus). Commented Oct 3, 2016 at 14:58
• As @orthocresol states, do some research first. I did this every year with my students. Do the "burning" in a plastic bottle of no more that 2 mL volume. I've done it with 4 mL and it's scary. You'll get a loud POP and the bottle will show traces of moisture inside. The explosion you experienced was water being created. But the water created was a gas so you probably saw no evidence of it. Also, as a liquid the water produced would be a very small volume compared to the gases used. Commented Oct 3, 2016 at 20:11

When hydrogen burns, pure water is created; the reaction is exactly the reverse of the electrolysis and shown in the equation below.

$$\ce{2 H2 + O2 -> 2 H2O}$$

If you just have hydrogen, then it will burn with a flame as you might know from a methane gas flame. However, once you have a certain mixture of hydrogen and oxygen gas in your container, the reaction becomes explosive. If you have the exact stoichiometric mixture of $2:1$, the reaction is a very strong (and loud) explosion; a tiny amount created a louder bang in Munich’s chemistry lecture hall than all other explosions the experimentors showed on that day.

The fact that hydrogen burns to give water and only pure water (vapour) was one of the main points used to try and sell hydrogen-powered cars, since water vapour is environmentally friendly and does not cause global heating like $\ce{CO2}$ does.

The electrolysis/explosion difference shows how different conditions for forwards and backwards reaction can be.

It bears repeating: do not attempt the combustion at home unless you know exactly what you are doing!

• Good points. Industry everywhere is looking at hydrogen-fueled combustion these days. One problem: hydrogen burns so hot it creates high nitrogen oxide concentrations in air. New combustion technologies have been developed to counteract that. Commented Oct 10, 2023 at 23:18
• Water vapor is a green house gas and is responsible for raising the temperature to livable level, changing CO2 is a modifier Also burning H2 in air also forms nitrogen oxides. Commented Oct 10, 2023 at 23:28
• @jimchmst water vapour is a greenhouse gas but one that is abundant in the atmosphere as is, that often reaches saturation levels (clouds, fog) and thus creating additional water barely influences global warming. This is especially true since most hydrogen gas is created from water by electrolysis. The same is decidedly not true for carbon dioxide.
– Jan
Commented Oct 18, 2023 at 9:56

Note that the reaction is exothermic so the water you make is going to be a gas. The reaction has a reasonably high kinetic barrier, so just plain heat is not enough. You need some free radicals from a spark or a flame to initialize the reaction.

Do be careful. This reaction is an explosion.