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Author: Subject: Heating Flammables in Test Tubes
bob800
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[*] posted on 5-3-2011 at 19:31
Heating Flammables in Test Tubes


In many small-scale experiments, a flammable material is heated in a test tube (by flame of course). Here's an example of an experiment I'm talking about :
<img src="http://www.sciencemadness.org/scipics/bob800/example.jpg">

It occurs to me that there are two possibles routes of an explosion:

1) There is a leak and the vapor comes in contact with the flame
2) The autoignition temperature is reached in the tube

What I'm confused about is route 2. Is it possible (or likely) that the temperature would get high enough to auto ignite the methane? In this scenario, it seems unlikely, as methane has an autoignition temp of 580C. However, another experiment of the same type involved heating sand moistened with ethanol to produce ethylene. Ethanol's autoignition point is 365C, which seems more possible for ignition.

Are experiments like this unsafe, or I'm I just over-thinking this? If the gas did ignite, do you think the glass would explode?
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Magpie
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[*] posted on 5-3-2011 at 21:40


This situation is common and with good containment poses no danger as there is no oxygen present.



The single most important condition for a successful synthesis is good mixing - Nicodem
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bfesser
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[*] posted on 5-3-2011 at 21:52


When I was young I had many glass tubes explode in my hands and face. As long as you use reasonably small tubes, you'll live and learn. ;)

Also, you may not want to deviate too much from the prescribed apparatus and procedures in 'chemistry set' type books. Some can indeed be dangerous if scaled up or changed.
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vulture
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[*] posted on 6-3-2011 at 07:38


Test tube "explosions" are often mislabeled as such when in reality structural failure through thermal shock is the culprit.



One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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unionised
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[*] posted on 6-3-2011 at 09:27


In that experiment, by the time the tube is hot enough to ignite methane, all the air will have been displaced so the methane can't burn.
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[*] posted on 6-3-2011 at 10:08


In the setup shown I would expect the stopper to be ejected rather than the tube shattered. Off course this really depends on what you're heating. But in general, I second bfesser: yeah, test tubes may possibly explode, if you heat flammable gases or unstable compounds. Use appropriate safety measures (goggles, apron, or additional depending on the materials) and amounts of material. Then it doesn't have to be such a big deal.
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Chaoschemist
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[*] posted on 6-3-2011 at 10:16


Quote:
Are experiments like this unsafe, or I'm I just over-thinking this? If the gas did ignite, do you think the glass would explode?

If you're heating a (relatively) low-boiling, combustible liquid, or like in the example up there, you're decomposing a substance to form a gaseous, combustible gas, the space "above" the hot area (and the hot area itself) will be flooded soon fully with this gas, replacing all the airborne oxygen from your apparatus. Now you won't have an ignitable mix of gas and oxygen in the "critical" area. So it's possible to decompose ethanol yielding ethene/diethylether, even if the ignition-temperature of any compound is reached.
It may be compareable to a protective-gas flooded apparature (like Nitrogen), but the danger is more the "worst case"-scenario like any breaks and leakages. So your gas will immediatly ignite if e.g. the flask breaks, if it's heated to ignition temperature or coming in contact with an open flame. But like the others already mentioned, keeping such an experiment "small scaled", will keep the dangers low, however giving the igniteable gases a possibility to accumulate, even a small-scale set up can become dangerous. So for example, it should be ok to boil off Hexane from a test tube while working under a fume hood, or directly ignite the coming-off vapours at the top of the test tube (Be careful about this too, your test tube may break due to the heat given off by the flame) Simply boiling may cause accumulation of combustible vapours waiting for any ignition source.
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bob800
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[*] posted on 6-3-2011 at 10:42


Thanks for all the reasonable advice. I'll keep quantities low as suggested.
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Magpie
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[*] posted on 6-3-2011 at 11:38


The next time you drive by an oil refinery look for the flare. Refinery operators have purposely set the escaping flammable gases on fire. It's a safety device.



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jsc
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[*] posted on 16-3-2011 at 09:44


As Chaoschemist says you will not have gases in the "critical area".

The temperature at the bottom of the tube where the flame is will be a lot higher than the top of the tube where the gases are.

Use a larger tube and put a thermometer in the gas area--you can see exactly how hot it gets.

Explosion/ignition risk is greatest when you have an empty vessel with just gases in it being heated. A nearly empty gas tank is a lot more dangerous than a full one.
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Magpie
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[*] posted on 16-3-2011 at 10:38


Quote: Originally posted by Magpie  
The next time you drive by an oil refinery look for the flare. Refinery operators have purposely set the escaping flammable gases on fire. It's a safety device.


Actually, any kind of gas combustion heater relies on this principle. Take a gas fired water heater, kitchen stove, camp stove, and barbeque grill as examples.




The single most important condition for a successful synthesis is good mixing - Nicodem
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