h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The change in air pressure caused by the matches will not be big enough to create that kind of pressure differential. Without doing the sums, I can't imagine that the change in density will be any more than 2 or 3 per cent.

What actually happens is that the burning matches use all the oxygen in the bottle (and, yes, they still use oxygen once they have apparently 'burned out'), creating a change in pressure of a whopping 20%.

B

From what I've read elsewhere, it is the temparature that makes the difference. The heat causes the air to expand and some of it forced out of the bottle. As the air inside cools it contracts and air is pushed back in from outside to fill the gap. Placing the egg in the neck of the bottle obviously stops this from happening until the greater external pressure forces it into the bottle.

You need to explain that the heat escapes; otherwise it makes little sense as the air pressure will actually increase.

No I think Mu B's explanation is better.

If the egg is plopped on the bottle top no air can get out, if it can then the pressure will equalise without any need for the egg to move.

The top needs to be sealed (by the egg) for any pressure difference to build up.

The reason the egg is sucked in is that the matches heat up the air in the bottle causing it to expand. When the air cools it contracts creating a suction effect.

To see how much suction is created get a plastic bottle i.e. soft drink bottle & fill it with very hot water, then empty it out & replace the top tightly, then run it under the cold tap and watch it implode.

Wow... Mu Beta... You really think that?

You're not the first. Science teachers have used this demonstration wrongly for quite a while. I believed the same thing until I saw Mr. Wizard (U.S. television science personality, recently died) explain the pressure/heating aspect.

After that I thought back to my science teachers explanation about the matches "burning" the oxygen and thought to myself "Durrrr".

The oxygen is used in the process only in the sense that it gets combined with carbon... but it doesn't get destroyed or go anywhere, (conservation of matter and all that).

The gas molecules inside the bottle expand greatly (think about it, their change in temperature on average is probably a few hundred degrees (Fahrenheit or Centigrade). Usually the experiment is performed by putting the egg on the bottle before the matches extinguish. The egg acts as a one-way valve allowing the heated gas to exit the bottle.Then, when the oxygen supply diminishes the flames go out in only a few seconds. The >POP< goes the egg into the bottle.

[Where's the edit button?] *THEN >POP< goes...

Er...yes, you must be right, and I'm being dim. It's because it's the school holidays. This is why I teach Chemistry and not Physics.

I'm on much firmer ground when I say that the vinegar does not _dissolve_ the egg shell; it reacts with it.

B

Yeah, I wrote a big long post agreeing with B earlier then thought about it for a while...



To be fair though, the oxygen will react with carbon mostly to produce charcoaly stuff, a lot of it will be sequestered but some CO, CO2 and H2O will be produced during combustion.

Yea, and that's what some science teachers probably think they're demonstrating while forgetting all about Boyle.

I think Im on pretty safe ground agreeing with B's original post.

Remember oxygen trapped in the burnt (oxidised) remains of the matches will be much denser and therefore have a much smaller volume than it would have in its gaseous state.

The matches will have the effect stated but not to extent necessary to cause the observed effects.

If that were the case the pressure created by the expansion of gas would without question equalize itself by popping the egg off the top of the bottle.

Without a one way valve allowing higher prssure air to escape the cooling of the air after the matches are fully extinguished would not cause a sufficient change in pressure to make the egg get sucked in.





Now Im thinknig abotu diesel engines and the pressure increase casued by rapid heating...

Theoretically both principles should and could be at work in the egg/bottle situation.

Infact they could be working in conjunction.

Typing as I think here Im not so sure now

I know.

Here's the experiment. Do everything as before just push the egg down a little into the bottle enough to seal the neck and then see if the expanding air has enough pressure to force the egg out or if the egg is sucked down you will know that any expansion due to the matches burning has less of an effect than the reduction in pressure due to oxygen being used up in the combustion.

If none of you do it I'll do this next week if we have nay rainy days on the half term play scheme.

Now Im recalling experiments in chemistry with tubes and burning materials where in a sealed system water traveelled up the tube towards the combustion due to reduction in pressure through oxygen depletion from the air in the system and not away from it due to gas expansion caused by temperature.



I could be wrong as I did a lot of chemical "experiments" of a different kind, which arent known for aiding memory, too

I know this thread is long dead but im bored so eh? what you going to do? whats this i hear about the combustion in the bottle after its sealed having an effect? assuming the match is basically made of long chain saturated hydrocarbons this baby is going to release like 50% more gas than its going to suck in. so the pressure would increase pushing the egg out. also assuming these matchs was pretting much nothing anyway its pretty clear from the gas law that theyre going to easily create enough gas to fill a 2L bottle or whatever this is. plus this gas is going to be like 200 deg c or more seen as its been part of an actual flame! the pressure change when this cools down as a rough guess would be just less than 0.7 bar. almost an entire atmosphere! if an egg doesnt get sucked into a bottle with this much pressure then im not really sure what it would need! so to clarify, burning matches would lead to an increase in moles of gas in the system and an increase in temperature so its going to be the subsequent change in temperature and pressure in the system that sucks the egg.

This all sounds like a bit of a miss understanding.
From the description the egg is only placed on the bottle after the matches have gone out. At this point there would be no pressure difference between inside and outside as there is no seal on the bottle.

The only difference would be that the air inside the bottle would have been heated and so have a lower density for the same pressure. The egg is then placed on the bottle, sealing it and as the air inside the bottle cools the pressure drops so creating the force to push the egg into the bottle.

Combustion definitely has an effect on pressure itself but not in this experiment because of the timing of when the egg is placed on the bottle. To prove that combustion actually reduces the gas pressure there is a good bar trick.
If you take an ash tray place a pound coin in the middle then fill add water to the ashtray ensuring the top of the coin does not get wet. then light a match place it on top of the coin and put a glass over the top. The effects will be that the match goes out when it runs out of oxygen and the water is all sucked inside the glass. You can check for bubbles while the match is lit but there is no gas escaping from the glass.

Hope that helps and the bar trick was a good puzzler for people as to how they get the water in the glass. Sadly with the smoking ban there's now no ash trays

Pete B

The egg is definitely placed on top of the bottle while the matches are still burning. The matches go out once the oxygen is sufficiently depleted. What's happening is that the heat expands the gasses in the bottle and most of the increased pressure is vented past the egg as it sits on the bottle opening. Once the flames go out, the gasses in the bottle cool and contract, quickly forming a seal with the egg before "sucking" it into the bottle.

Otherwise though it's as you describe.

I wonder whether the egg is really just sad and cold, and sees the nice warm bottle as a good place to go?