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The Delights of Chemistry
Dan Ormsby writes: "No news on this site, just great photos of chemical phenomena along with instructions on how to perform them yourself. Don't try this at home!"
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Check this:
LNUX + IPO = $0
bitches
Why is Slashdot so slow all of the sudden? What is with this "Nothing for you to see here, move along" crap?
Better living through cocaine. Artificially induced libertarianism, as it were.
Demonstrations 04 & 05
Flammable vapours
Continuing the theme of combustion this is an attempt to model an incident that occurred in a small garage local to where MH lives in Sheffield. In these spectacular experiments dihydrogen, diethyl ether and alcohol are burned.
Liquids that vaporise easily at ambient temperature to form heavy vapours pose particular hazards during handling e.g. when transferring from one container to another, or when spilled.
Petrol is the most common of these fluids and serious accidents have occurred during handling due to flashback from sources of ignition often at considerable distance away. Because the heavy vapour spreads along the ground it does not readily defuse into the air and become diluted below its explosive/flammable limit so the, even outdoors, it can be very dangerous. People attempting to kindle garden fires or BBQ's with petrol have all too often been engulfed in flames due to the spread of the vapour beyond the intended area. Similarly liquid petroleum gas (LPG) has also caused appalling accidents when 'fire balls' have formed due to the ignition of dense clouds of the gas. Butane has a density of more than twice that of air.
These experiments attempt to simulate, and so demonstrate, this phenomenon. Diethyl ether is used because it has a combination of high vapour pressure (440mm Hg at 20C) and density (2.55 {air = 1}) and has been responsible for many accidents in the laboratory and the chemical industry where it is often used as a solvent in chemical processes.
In the first demonstration a length of extruded aluminium angle is set up to form a channel along the length of the lecture bench (some five meters) such that one end is at bench level and the other some 150cm higher. In the video this is said to represent the floor of a garage (set sloping in order to reduce the experimentation time) and a wad of burning cotton wool soaked with a little alcohol at the lower end is said to represent someone welding (a source of ignition). Balloons are attached at the high end. These are filled with hydrogen gas except the yellow one to which a little oxygen had been added in addition to the hydrogen (the story line has these to emphasise the effect of a leaking fuel tank exploding). Sufficient diethyl ether wars added to the high end of the channel (representing a petrol leak) such that it was all vaporised by the time it had reached about half way down. The heavy vapour continued down the remaining 2.5m and was ignited on reaching the 'welder'. The thunderous sound of the explosion it impossible to reproduce from the video but some compensation is afforded if the exploding balloons are played back frame by frame.
The speed of the (defusion) flame front along the channel was quite slow due to the lack of mixing of the vapour with the air. In reality air movement, particularly if it were outdoors, would pre mix some of the ether and air. The second experiment attempts to simulate this. The storyline here is that the space enclosed by the Perspex safety screens represents a laboratory and the candle represents a Bunsen burner. Diethyl ether (15ml) is spilled on the floor a metal tray and left to evaporate. Later a small electric fan is started to create a draught and so mix the vapour and the air. When this pre-mixture reaches the source of ignition the resulting conflagration is very rapid and if were much larger or confined, an explosion would occur. See also experiments 7 and 15a.
This is one occasion when seeing it in real time is less impressive than the series of single frames, there are 25 each second. The series of diagonal stripes from top left to bottom right, to be seen on one of the frames, are vertical, or near vertical, oscillations (pressure regions). They are slanting because of the way the image is scanned by the camera, left to right, top to bottom.
By using video frames as units of time (each represents 40ms) the relative velocities of the near diffusion flame-front and the pre-mixed flame-front can be, roughly, determined. Since the same synchronisation pulse as received by all three cameras any combination could be used for this purpose. I used camera three for the (near diffusion) flame on the channel and counted the number of frames that showed it passing the front of the safety screen enclosure. The images on the video are from another camera and are not useful for this. I used frames from camera one for the pre-mixed flame-front which is the one use for the video at that time. I got the following results:
Number of frames taken for the 'diffusion' flame to traverse the enclosure front = 17 (= 680ms)
Number of frames taken for the pre-mixed flame to traverse the enclosure front = 7 (= 280ms)
Relative velocities 680/280 = 2.4.
So the pre-mixed flame travelled about two and a half time faster than the diffusion flame. However, until the flame was bounded by the walls of the enclosure the increase in volume was 2.43 faster i.e. about 14 times!
HAZARDS
The main hazards are exemplified by the demonstration itself. . Diethyl ether has a high vapour pressure (440mm Hg at 20C) and density (2.55 {air = 1}) and a low auto ignition temperature (not flash point!) of between 180 and 190C. Air/ether mixtures containing >1.85% v/v of diethyl ether vapour are constitute an explosive hazard.
Explosive peroxides (ethylidene peroxide polymers') may be formed on exposure to air and light, and particularly when evaporated to dryness. These peroxides have been known to form around the stoppers of container of diethyl ether that have been left unopened for long periods. Although not relevant to this experiment it is worth pointing out that ether is a good electrical insulator and when very dry (saturated ether contains 1.2% water at 20C), shaking etc. can produce sufficient static electricity to cause ignition of the vapour.
Diethyl ether is mildly irritating to skin and mucous membranes. Inhalation of high concentrations causes narcosis1.
Hydrogen gas is very flammable and yields explosive mixtures with air. The stoichiometric mixture should be avoided because the explosion is too loud. The audience should be, at least, three meters from the balloons.
DISPOSAL
Immediately after the explosion a careful check should be made to make sure that no debris from the balloons are burning2. I have performed this demonstration over 70 times (>280 balloons) and have not found this to happen but one should be aware that it could! The cotton wool should be extinguished by covering with a watch glass and left to cool.
References.
1. P. G. Stecher, (Editor) The Merk Index Eighth edition, 1968 p. 483
2. B. Iddon, The Magic of Chemistry BDH Product Code: 57045 ISNN: 0 9800439 6 6 1985 p. ii
I mean, I'm all for Chemistry and all that, since the physical sciences pay for my grant, but this is just a random university website: why is this front page worthy? Was there no Microsoft news today? Or is this symptomatic of a general decline in the tech sector?
"I will take the Ring," he said, "though I do not know the way."
Stuff the matters
Blowing shit up matters. Trust me.
Let's post really obscure articles with absolutely no details, that way we can be sure the /. effect kicks in asap...
Think outside the... Hey, where'd the friggin' box go?
Science: The Delights of Chemistry
Posted by Hemos on Thursday August 30, @03:15PM
from the ka-RASH dept.
Dan Ormsby writes: "No news on this site, just great photos of chemical phenomena along with instructions on how to bring a website to a grinding halt. Don't try this at home!"
jcase@serv.net
ahknight@pobox.com
comp-u-geek.net link in that cache!
Enough with the scientology crap, dingdong. Don't post copyright materials here.
Can someone kindly explain how the moderating system works. Didn't quite know where else to ask this question. Sorry if I broke up the thread here.
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29jan0
REDUNDANT?!!! I'd call it OFFTOPIC, but I guess moderator knows better what he's up to ;)
even though it says dont try this at home, i did.. and well, i burnt my penis off and my balls shrunk to something resembling raisins. who do i sue?
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