Take the 2-minute tour ×
Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers and students. It's 100% free, no registration required.

I want to electrolyze water to get oxygen and hydrogen. I want to mix them in a regular balloon and ignite it. How much hydrogen would you need in a 2:1 ratio with air for it to be dangerous? Is this generally a safe experiment?

share|improve this question

4 Answers 4

up vote 6 down vote accepted

In case FriendofKim can still read this...

First, let me say that I've enjoyed many times exploding soap bubbles of about one milliliter filled with hydrolysis gas. That is 1 cubic centimeter. That will give you a sound that rings in your ears in a decent sized living room. You may wish to use ear protection for the experiment.

50 ml will have an effect in a lecture hall that not only wakes up everyone, but also may make people complain.

Now while the explosive limits of hydrogen in air range from about 18 -- 60 % the flammable limits are from 4 -- 75 %, in oxygen the limit of flammability goes all the way from 4% to 95% read: for practical purposes, hydrogen in oxygen is always at least a flammable mixture.
For comparison, gasoline in air is flammable roughly between 1.5 - 7%.

However, you start with a stochiometric mixture which is ideal for explosion (after all, that's why you do it, right?), and it is in oxygen, not in air. So even if you "only" have a flammable mixture reaching an ignition source (e.g. electric switch), chances are quite high that as the ignition proceeds through the mixture, it can reach a zone where the gas/air mixture is explosive. Also, because hydrogen is so much lighter than air, it tends to accumulate under the ceiling, so even if there is enough air in the room that an ideal mixture would not be explosive, chances are that there is an explosive layer of gas. (Same with gasoline or, practially more relevant: solvent from glue for parquet floor tiles)

The safety relevant points are:

  • the limits of explosion and flammability are, even in air, extremely wide.
  • limits of flammability are important even if there happens to be no explosive mixture: hydrogen is a small and light molecule that diffuses fast. If the air you breathe happens to have a hydrogen concentration within the flammability limits, any ignition will burn deep into your throat on its way to the lungs.
share|improve this answer
    
I'm actually planning on making very little of this just to get a little sound. I've seen people ignite large party balloons with hydrogen before. Are you saying it could have been a big disaster? –  Friend of Kim Feb 15 at 21:14
    
It could, but there is one thing that usually makes the balloon experiment less dangerous (and IMHO somewhat misleading as to what a good explosion is): the hydrogen diffuses really fast through the rubber of the balloon, while the oxgen stays in. As it takes a while to fill the balloon, the resulting mixture is far from ideal. –  cbeleites Feb 15 at 21:16
    
So you're saying that there is very little hydrogen left, and thus making the explosion less intense? –  Friend of Kim Feb 15 at 21:18
    
Yes. I've also had school chemistry lecture hydrolysis gas explosion. After the soap bubble experiment I can tell from the sound that the mixture was not ideal. The trick with the soap bubbles is that this is really fast. We made them 2 - 3 per second with an autogenous welding machine for dental technicians. It uses a canule as tip, and we just put that into soap water and immediately lighted the bubbles. When not bubbling, we kept the welding torch burning - that is the safest way. –  cbeleites Feb 15 at 21:19
    
Aha, okay. I'm only intending to make a very small amount, anyway. Thank you for pointing this out. –  Friend of Kim Feb 15 at 21:20

UVic, three party balloons, each to be tied to a ring stand: 1) Pure hydrogen. Flame on balloon. Whoosh, it burns. 2) Hydrogen plus demonstrator's breath to inflate. Demonstrator puts on helmet, ignites from a yardstick. BOOM!!! Dust falls from ceiling. 3) Hydrogen plus oxygen 2:1. I'm about 50 feet back, in the vary last row of a filled auditorium, snugged in the corner of the AV extension. It puffed me. DON'T DO IT.

share|improve this answer
    
Thatt sounds a bit scary. I'll make only very small amounts then. My goal isn't to blow something up, only to get a puff. –  Friend of Kim Feb 15 at 9:26
    
I now think 15 liters sound a bit much. I was thinking about about a regular balloon, and by searching Google found 15 liters which sound a bit much now. –  Friend of Kim Feb 15 at 10:24
    
How's your hearing aid? –  cbeleites Feb 15 at 21:11

Enough that I wouldn't recommend trying a home-experiment.

Explosive limits are ca. 18 -- 60 % in air, flammable limits 4 -- 75 %.

In all honesty you'd probably be fine, as long as you're careful. I personally wouldn't do this at home or anywhere near something I didn't want near an explosion, and I'd still make sure you were behind something when you started it.

I also have a question for you:

Have you ever heard of the Hindenburg?

share|improve this answer
    
Thank you fo your answer! I'm talking small amounts here. Only enough to fill a balloon. Would it be safe to do outside? –  Friend of Kim Feb 15 at 1:57
    
I've never done it personally, so I can't be positive. Probably, but again - take all safety precautions. –  Dannnno Feb 15 at 2:20
    
Yes, I will. I'm thinking about making a deciliter and test a bit. Then decide if making a whole liter is safe. –  Friend of Kim Feb 15 at 11:51
    
Fill a balloon does not count as small when hydrogen is concerned! –  cbeleites Feb 15 at 19:55
    
@Dannnno: please see my edit - the explosive limits you gave were completely and dangerously wrong. I'll write an answer where I explain them more in detail. –  cbeleites Feb 15 at 20:05

Here is a neat experiment, but it unfortunately needs a welder to make the equipment, a metal sphere with a volume of about 100 mL. It should have a very small opening on top and a larger one on the bottom. It also needs feet to stand.

experiment setup

The metal container should be capable of some pressure, because the gas will explode inside and you don't want to hurt anyone. Additional you need bucket or bowl filled with water and hydrogen gas.

First, close the hole on the top of the container and place it in the bucket under water until it is completely full. Now you can fill the container with hydrogen gas until all water is gone from it. When you remove your container, make sure that the bottom hole stays on the bottom. As Hydrogen is much lighter than air, it will be trapped inside.

Place the container on a stable, nonburnable table/ bench. Have a lighter ready. Now open the top hole and ignite the hydrogen gas. Move away from the experiment. After a while you will hear a phenomenal boom (make sure you have your mouth open). Do not try this in a small room.

The trick behind this is in the ratios. As Hydrogen will burn on top, air will be sucked in at the bottom. As long as the mixture has not the right ratio, it will not explode but just burn at the top. Once enough oxygen has entered it will ignite the whole thing and do a phenomenal bang. The experiment is pretty safe.

The experiment is part of this lecture, which is in German unfortunately.

share|improve this answer
    
And this is with only 4 percent of oxygen. I wonder what would happen with a 1 to 2 ratio... –  Friend of Kim Mar 26 at 10:47
    
@Friendofkim According to this the 'concentration' of hydrogen in air should be about 59% when it explodes (you start at 100%). So Oxygen level should be about 8%. Nevertheless, if you have a 1:2 mix then cover your ears and keep your mouth open! –  Martin Mar 27 at 2:41
1  
Ahh, I wrote the comment a bit too fast.. I was thinking about the burning percentage. –  Friend of Kim Mar 27 at 12:56

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.