# What is the safe level of energy output for hydrogen combustion at home?

With confinement, my son would like to do some home experiments for a class. So far, he has a good setup that seems sound for the electrolysis of water. A later part of the experiment would combine back the $$\ce{O2}$$ and $$\ce{H2}$$ to get back water vapour.

What amount of atmospheric pressure $$\ce{H2}$$ and $$\ce{O2}$$ can be safely combined, at home, without it turning to a fireball and/or a news event? From Googling, I found 12,749 joules per litre of hydrogen. What I'm asking is:

How does dissipating 12K joules look like in a plastic bucket? Is this a case of needing only gloves and eye protection, or does it need further protection? Will it be just some heat, just a small flame, or a McGyver-type explosion?

Also, will $$\ce{H2}$$ and $$\ce{O2}$$ at atmospheric conditions instantly react, or would this be a progressive reaction?

• Certainly don't do it on a large scale. Small amounts of hydrogen oxygen mixtures from electrolysis bubbled through soapy water in a standard bucket will create a big enough bang when ignited to be troublesome indoors. Even perhaps 100mL of bubbles will be a big bang. – matt_black May 26 '20 at 17:40
• I'm not as familiar with this SE as some others. I understand my question might not be correct as is. Down-voters, if you would please comment, in addition to down-voting, that would give me the opportunity to fix the question. – Jeffrey May 26 '20 at 17:43
• I did not downvote, but I have heard the sharp retort from a small balloon filled with hydrogen and oxygen. The small soap bubbles method suggested by @matt_black is the way too go: no fast moving fragments of significance. As for downvotes, things that blow up are problematic for a variety of reasons, as you can well suppose, and safety must be paramount. – Ed V May 26 '20 at 18:50
• I remember a high school experiment presentation in early 80s with bubbles from O2+H2 mixture in a china bowl. It crashed the bowl, making from the fragments a kind of a geometric projection on the table. – Poutnik May 28 '20 at 7:08

First things first: hydrogen gas and oxygen gas do not spontaneously react but need activation. Activation can be provided by a flame, an electric spark or similar.

That said, here is the important part. The reaction will not turn into a fireball but even small volumes will explode with pretty loud bangs. It’s not the loudest bang known to chemistry instructors but it’s certainly something to be aware of – you don’t want your neighbours ringing the police because they think a shooting has been going on.

If you’re going to try this, I recommend starting with the smallest volumes possible, not bigger than a shot glass. My chemistry teacher in high school used a glass cylinder approximately the size of a small (200 ml) water glass. That gave an even stronger bang but the cylinder itself remained unharmed, probably because there was a large opening on one side.

However, the experiment is unlikely to show you much because the water vapour is obviously gaseous and will dissipate rather than being visible anywhere. It would almost be more instructive to identify the two gases as hydrogen (burns when brought to a flame) and oxygen (ignites glimming wood) by the standard procedures used in high school (at least in mine).

Tl;dr: I suggest you don’t try igniting a 2:1 mixture unless you’ve seen it performed recently and know what you’re doing.