Gas and coca tin experiement

[mike]

Years ago (60's) at school I recall an "experiment" where the teacher placed a rubber gas tube into the base of a coca tin. The top of the tin was tight on, and had a hole punched in it. The gas was turned on and the top of the tin was ignited. A flame burned there. Then, he turned the gas tap off and waited... A few seconds later there was an almighty bang and the lid blew off, hit the ceiling and flew into us kids. No one was hurt. I suppose the gas:air ratio was explosive at some value.

Why/(not not) does/(does not) this happen if, in a house, someone turns the main gas tap off while (say) a gas fire is lit? The only reason I can think of is that the gas jets and pipework are too small an orifice to allow this to ratio to develop. Yes/No? If not why not . Answers for homework before Friday :-)

ps to the unenlightened DO NOT TRY THIS - ANYWHERE

[Peter Crosland]

I did not know coca came in tins!

[Chris Hogg]

As in

Two points. First, in the exploding coffee/cocoa can, air is drawn in through a bigger hole in the base, so mixing with the gas in the can until at some point an explosive mix is formed. This does not happen in the gas supply pipes, as the tap seals the pipework from the outside air so preventing any back-mixing.

Second, there is a small opportunity for it to happen within the mixing tube of, for example, a gas ring, where air mixes with the gas from the jet, before emerging through the ring of holes that form the burner itself. It's been a long time since I turned off a gas ring, but IIRC there was always a small 'pop' just after the gas was turned off, as the flame died away, which I assume has exactly the same cause as the exploding coffee tin but on a smaller and more contained scale: the flame disappears through the holes in the burner ring and ignites the now-explosive mix in the mixing tube.

[Alan (BigAl)]

That is right. You did get the "pop" when you turned off the gas. The old gas stove burners had the nozzle some way back from the burners and an air vent for the creation of the right mixture of gas:air

The Bunsen burners would do that too.

[Brian Gaff]

Because air always gets in from a different place I imagine. after all your Bunsen burner never blew up either did it? The experiment as I recall it was an upside down tin with a hole in the bottom withe a finger over it. shoot the gass in the open bottom and then with a little vasaline in between to stop leaks, put it on a surface and light the hole with finger now removed, stand back and said tin goes bang and hits ceiling of lab. Brian

[Brian Gaff]

You are changing the subject. The fact is, that blowbacks can occur, but not very often. Witness those coin in the slot meters of old times. these often went out when the gas cooker was on, and there were no bangs. I suspect there was no way for the mixture to be made explosive. However, in rare cases where the fault is quite a way back and there is an underground leak, you do get a bang. This has been known to demolish houses and make holes in roads.

Incidentally, have you ver noticed that gas men use lit flames to detect gas leaks. They tell me its safe in free air outside as an explosive mixture cannot build up and they can see by the colour of the flame where there is gas present. Brian

[Caecilius]

I remember that one as well!

As other have already said, it's about air mixing with the gas to create an explosive mixture.

It was also much easier to do this experiment with the coal gas (town gas) that was used in the 1960s. I've not seen it work with methane (natural gas). I think the hydrogen content in town gas was more likely to create an explosive mixture with air, as the explosive range for hydrogen and air is very wide, whereas methane/air is narrower.

[Huge]

Write idea, wrong reason. Town gas was lighter than air, so rose through the hole in the top and dragged air in the bottom, until an explosive mixture was formed. Natural gas is heavier than air, so it doesn't work.

[Malcolm]

Natural Gas is not heavier than air (perhaps you are thinking of propane or butane - bottled gas- which are heavier than air). It is about half as heavy as air whereas town gas was probably less than a quarter as heavy as air.

Malcolm - a graduate chemist

[Dave Liquorice]

Hum LPG (butane and/or propane) is heavier than air but methane isn't, from:

Density in kg/m^3 at STP.

Hydrogen 0.090 Coal gas 0.580 Natural Gas 0.7 - 0.9 Methane 0.717 Air 1.293 Propane 1.882 (NTP) Butane 2.500

Natural Gas is not as bouyant as Coal Gas but it's still bouyant. Wonder why the experiment doesn't work? If we had mains gas ...

[Tim+]

Think the first explanation is right. Natural gas is lighter than air.

Tim

[Graham.]

We did that experiment at school, and in collage a few years later I brought in a 4 ft dia metrological balloon which we filled with coal gas and pushed out of the lab window. We did conceder attaching a postcard, but our lecturer didn't think that was such a good idea ;-)

[Weatherlawyer]

It was the air mixture that provided the explosivity. Today we have air-burst rounds to destroy property using the same method. It was a perennial problem in mines and freighters where inflammable dust might be ignited.

With a Bunsen burner, if the air was mixed with the gas it burned with a blue flame if the air inlet was closed it burned with an orange flame. Under a busy stove this could cause the mixture to pop when the flow was slowed or stopped.

[Tim Streater]

It would have been if you'd set light to the postcard before launching the balloon.

[Huge]

Bum. You are of course, correct. Because we only have bottled gas here, it slipped my mind that natural is mostly methane.

Although the willy waving is unnecessary & pointless on Usenet.

[Huge]

Yes, yes, I get it. :o)

[John Rumm]

I remember our physics teacher attempting to do that demo, but it never worked since we did not have mains gas and LPG is heavier than air, so it just fell out of the hole in the bottom of the tin!

[Tim+]

Well he should have lit the hole at the bottom then! I think the reason it doesn't work these days is down to the lack of hydrogen as suggested by an earlier respondent.

I feel an experiment with tin foil, caustic soda and a cocoa tin coming on... ;-)

Tim

[Tim Lamb]

Tate and Lyle golden syrup tin when I was growing up. You need a tight fitting lid. Coal gas then of course.

>

[Vir Campestris]

Slight whoosh! there I think.

Cocoa often comes in tins. Coca grows on bushes...

I recall using a large catering size bean can from the school kitchen, perhaps 9 inches high and fat to match. The "owner" had fitted a nozzle on one side, and a diaphragm on the opposite side to make a sound cannon. It would blow out bunsens from several feet away. It then occurred to me to find out what would happen if it had gas in.

It didn't pop, but it did produce quite a nice flame jet before the diaphragm melted. It was slow enough that I could put it down before that point - so the diaphragm didn't melt onto my finger tips.

Andy