I guess some of the responders missed my previous message regarding this demonstration. For that reason, I'm repeating it here with some additional information.
Burning hydrogen is a test tube is, in my opinion, a better and safer demonstration that exploding hydrogen and hydrogen-oxygen balloons. There is a lot of pedagogical value in this demonstration.
The hydrogen is generated by reacting a metal, often magnesium, with acid, usually hydrochloric acid. The acid does not have to be more concentrated than 3M. This shows a single replacement reaction of hydrogen by a metal. If properly done, only a small
piece of magnesium is needed, one or two grams, and only about 20 mL of the acid. The amounts will vary depending on the size of the test tube used to capture the hydrogen gas. Without any large excess of the magnesium, there should be only a little excess
of hydrogen gas generated into air. If more hydrogen gas is needed for a second reaction, only another small piece of magnesium metal should be used with a fresh, dry test tube to capture the gas. Normally, when I demonstrate this to a class, there is only
enough magnesium added to the acid to generate sufficient hydrogen to fill the collection tube.
The hydrogen gas is captured in an inverted test tube held over the reaction tube. This illustrates the hydrogen is less dense than air.
In the inverted test tube is kept inverted and moved away from the reaction tube to a burning candle or a Bunsen burner, at a safe distance from the generating test tube, it illustrates that the hydrogen gas does not escape from the tube and reinforces
the observation of hydrogen being less dense than air.
When the open mouth of the test tube is placed near the flame, the reaction will vary depending on the hydrogen-air mixture in the inverted test tube. With sufficient air, there is a "pop" or explosion. Since the test tube opening is not constrained,
there is not any large pressure created. If the test tube is filled with almost pure hydrogen, a slow burning of the hydrogen will occur as it mixes with air. I prefer the second option as it can be observed that the inside wall of the test tube becomes
coated with condensed water from the reaction as the flame moves through the tube. In either case, there will be condensed water inside the test tube.. This illustrates the direct union of hydrogen with oxygen from the air.
The experimenter must wear the proper PPE. A lab coat and safety goggles. There must be good ventilation in the room. The test tubes must be inspected for any chips or cracks. I prefer heavy wall ignition tubes, if available. The test tube used to generate
the hydrogen gas should be supported in a test tube rack or clamped. The collection tube should be held by a test tube holder or clamp - never in bare hands.
I have performed this demonstration hundreds of times in my career. This reaction was also part of laboratory experiments on reactions and/or periodic properties in commercial lab manuals In my experience, there were never any accidents doing this reaction.
That does not mean that an accident is not possible, but I believe, if performed properly, it is of low probability.
I have attended many public demonstration programs where the demonstrator explodes multiple hydrogen and hydrogen-oxygen filled balloons. Often, the audience is not warned about the loud explosion. There is no physical evidence of the formation of water
visible. Other than showing that hydrogen is flammable, I do not view these explosions as learning experiences. If one wants to show hydrogen explosions, videos of the Hindenburg are posted on YouTube.
Since 2M or 3M hydrochloric acid is involved, it is up to the demonstrator/experimenter if the appropriate type of gloves should be worn. Thermal gloves are not required as the tube should not be held with bare hands and the tube does not get hot.
This demonstration/experiment is meant to be performed on a small scale. The only oxygen in the tube, used to collect the hydrogen, is from the air. I agree with Daniel Crowl that this should not be performed in any large tube or pipe especially with
pure oxygen added to produce a louder "pop".
Jarral, if the test tube is filled with almost pure hydrogen, there will not be a "pop" and, in subdued light, one can observe the hydrogen burning in the tube and the water condensing on its walls. It took me a few tries, but I was able to photograph
it.
The late Hubert Alyea did this reaction in almost all of his presentations. He used the old type of glass Coca-Cola bottles (about 10 to 12 ounces) which he referred to as "pop" bottles. The bottles contained a mixture of hydrogen and oxygen gases and
about 5 mL of water stoppered with a cork. (I'm not sure of the composition of the gas mixture, but it was about 2:1 hydrogen to oxygen.) The bottles were stored with the cork end down with the water in the neck of the bottle so it was obvious if the gases
in the bottles leaked out. After he ignited the gas mixture, he poured out the water which he claimed was the product of the reaction. A true statement, but not that much water was actually formed.
Hubert also cheated on his burning candle in an enclosed space demo. He would fill a beaker approximately 20% full of water and pour it into a shallow bowl containing a candle. He would then light the candle and invert the beaker over it. When the candle
burned out, the water had risen up about 20% of the volume of the beaker. It worked every time!
As many of my CHAS colleagues have stated, in the past, you must know what your are doing when you do an demonstration or experiment. I have always stated that bigger in NOT better. Scaling up any experiment is not a simple procedure as there are a number
of factors involved - that's why there are chemical engineers who can do that.
Best regards to all for a happy, healthy, and safe New Year!
David
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----- Original Message -----
Sent: Sunday, January 3, 2021 1:05 PM
Subject: Re: [DCHAS-L] "Hydrogen pop" demo and PPE
Ok. Get on with it..... this has been done for decades (century?) in school science classes. Any one ever hear of an accident? Like we see with burning methanol with metal salts? Is saying a hundred more times what the ppe needed going anywhere at this
point? Or that you shouldn't do it? People are going to do it.
I did it way way back. It was an obvious test for hydrogen. CO2 gasses made, put out flames when putting a burning splint in. Kids are doing it now in freshman science at our public school. I asked.
Sorry to vent....
As far as the original question. A picture of the flame with very fast timing or time exposure could look cool. The flame probably would be hard to see though... of course do it behind an explosion shield. I guess that was the original thought to take
a better picture?
Jarral Ryter
Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------
From: Daniel Crowl <crowl**At_Symbol_Here**MTU.EDU>
Date: 1/3/21 10:38 AM (GMT-07:00)
To: DCHAS-L**At_Symbol_Here**Princeton.EDU
Subject: Re: [DCHAS-L] "Hydrogen pop" demo and PPE
NOTICE: This email originated from outside of the University. Do not click links or open attachments unless you recognize the sender and know the content is safe. Thank you, Western IT Services.
Hi All,
I would like to comment on the hydrogen POP demo and warn everyone about the possibility of a hydrogen detonation.
I was on the ASTM E27 committee for many years - this committee handles all the ASTM flammability standards - and there was always a continuing discussion about hydrogen making a transition from a deflagration
to a detonation.
The demo video that I watched was of a hydrogen deflagration - with the combustion occurring at a speed less than the speed of sound. It is possible for hydrogen to make a transition to a detonation - with
the combustion burning at or greater than the speed of sound - upon which the glass tube would immediately explosively shatter, even with the open end.
Hydrogen is prone to this phenomenon since it burns much faster than other gases. This is demonstrated by a bigger "pop" with hydrogen than, lets say, methane.
Most of the evidence of a hydrogen detonation is from anecdotal case histories. No one knows how to even construct an experiment to study this.
What is known about this phenomenon is:
1. It is more likely to occur in tubes or pipes with one closed end. The closed end causes pressure piling and the transition to a detonation. The glass tube in the demonstration is closed at one end.
A louder "pop" is obtained in the demo with the closed end due to this pressure piling.
2. It is more likely to occur with enriched oxygen since this increases the burning speed of the hydrogen.
3. The longer the tube or pipe the more likely the transition will occur.
4. It is not clear how the tube diameter affects this.
I would say that the probability of a transition to detonation is unlikely for this demo, as shown in the video.
However, I can see someone increasing the tube length and using oxygen to increase the "pop."
Why is this demo even being done? What does it demonstrate? I realize it has a high "fun factor."
Dan Crowl
Emeritus Professor, Michigan Tech University.
Fire, glass and chemicals? Is there really a question about what should be worn?
Hi all--I'd appreciate any thoughts on someone wearing just eye protection for this demo. On the one hand, it's a small amount and the test tubes don't have constricted necks. On the other hand, it looks like a glass tube held in bare hands
near the face.
https://twitter.com/I_am_Endeavour/status/1340788101283803140
It's been suggested as something to post in C&EN's
"Chemistry in Pictures" collection for New Year's Eve, and I need to decide by tomorrow (Wednesday 12/30) whether to go ahead.
Thanks for your input!
Jyllian
Jyllian Kemsley, PhD [she/her/hers]
Executive Editor, policy and content partnerships
Chemical & Engineering News
M: (+1) 925-519-6681 | Skype: jyllian.kemsley
Twitter: **At_Symbol_Here**jkemsley
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