Chem Riddles. Number 6

Reprint from Chem 13 News, February 2001, page 4

Yehoshua Sivan  
Menachem Begin High School
Safed, Israel

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Chemistry Riddle #6.   The hovering flame

If you unscrew the barrel of a bunsen burner, you might be able to light a tiny flame at the orifice at the base of the barrel.  By increasing the flow very slightly, the flame will hover.  I use a portable Labogaz condensed butane burner, and have no difficulty with the demonstration.  (These burners are readily available in Europe and Asia — an internet search using MetaCrawler gives suppliers in the UK, Switzerland, Germany and Singapore.  Readers in other countries should be able to find similar burners in stores that sell camping and outdoor equipment.)

four Bunsen Burners (first with valve and not lit, second has no valve and not lit, third is lit and fourth has flame hovering)

1. A regular portable burner (butane)

2. Unscrew the head (Take care! Do not unscrew the bottom part)

3. Light a tiny flame.

4. Increase flow slightly -- the flame hovers. 

This demonstration (or activity – there’s no reason why students shouldn’t play around with the burner themselves, and develop some motor skills!) never fails to fascinate people of all ages.

The riddle posed is "Why does the flame hover?"

Didactic points

Students must be familiar with the concept of the “triangle of fire”.  They will have to analyse the phenomenon with the triangle in mind.

The problem is not why the flame burns, but why it doesn’t burn (between the orifice and the hovering flame).

There’s no shortage of gas (fuel), for otherwise the flame would not burn at all.

There’s no shortage of air, both for the same reason, and anyway it’s obvious that air surrounds the burner.

There doesn’t appear to be a shortage of heat – the flame is hot enough to ignite a match or a piece of paper.  For all that, it fails to ignite the gas below it.

Ultimately students have to come to the conclusion that the gas itself is too cold to be ignited, until it reaches a certain point, where the flame begins.

The question now is: Why is the gas cool as it emerges?

The solution

Teachers will probably be familiar with the Joule-Thomson effect: as the pressure of a gas falls while streaming through a small hole, so its temperature drops.  The energy needed to overcome the attraction of the molecules is taken from the molecules themselves.  These cooled gas molecules warm up in the couple of centimetres between the orifice and the flame, presumably by contact with the air.

Students may find it easier to derive this concept by comparison with the self cooling effect during evaporation of a liquid such as acetone.  If its temperature is measured while still in a closed flask (a thermometer can be inserted via a hole in the stopper), then when it is poured into a wide beaker, the temperature will be seen to drop as the acetone evaporates.  Again, the energy needed to overcome the attraction of the molecules is taken from the molecules themselves.

Indeed this phenomenon of evaporative cooling might be partly responsible for the low temperature of the butane emerging from the orifice if the burner is used over a prolonged period of time.  The cartridge holds liquid butane, and if a burner is used for a long time the cartridge becomes remarkably cold, both in the liquid phase and the compressed vapour phase.  (This, in itself, is material for a riddle!)   However, since the hovering phenomenon occurs when the flow is first turned on, before there is any significant evaporative cooling in the contents of the cartridge, the major cooling effect must be due to Joule-Thomson cooling.

Added note.  One reader of this article suggests an alternative explanation — that mixing of air and fuel is not adequate to give ignition until the fuel/air mixture is several centimetres above the orifice.  But, at slow flow rates mixing can’t be a problem because the flame does ‘sit’ on the orifice when flow is slow.  Also, in a Bunsen burner, when the air supply is sealed off, the flame sits right on the orifice;  here too, then, mixing must be adequate.  I prefer the explanation in terms of heat supply, but I would be happy to hear from readers who know more about this phenomenon.   

Want another of Yehoshua Sivan's riddles? Riddle 1