When I light a match, drop it into a jar and place a peeled, hard-boiled egg over the opening of the jar, the egg is pushed into the jar. I understand that the air pressure inside the jar has changed but I don’t know why. Is it the density of the air inside the jar versus the outside?
I’ve tried to search online for an explanation but most only get this specific:
Thanks!
Hot air expands, so it is less dense than cooler air. i.e, a given mass of air will take up more space when heated. When the air in the jar cools it creates a vacuum, so the air outside pushes the egg in.
The explanation you posted sounds pretty clear to me. As the hot air inside expands, it pushes the egg out of the way so some of it can leave. This leaves less air inside - that is, there are fewer gas molecules in the jar than there were. As what’s left cools, it takes up less volume. This causes a pressure difference. The inside of the jar is “missing” all those molecules that left, and needs them back. Eventually, the difference is large enough that the pressure of the air outside can shove the egg through the hole.
Incidentally, this is a demonstration of Charles’s Law.
So the heated air expands out of the top of the jar. As the temperature decreases the volume decreases (and the air can’t immediately get back in, due to the egg-plug), creating the vacuum.
Thanks so much! Now I can help my 6-year-old with his science fair project!
You have to think of the egg as a one-way valve (i.e., a “check valve”) that lets the hot air out, but doesn’t let it back in later…TRM
You’ll probably have to argue with those who believe that the flame creates a vacuum by burning away all the oxygen. I remember arguing with my third grade teacher about this. As I recall, she seemed to be under the impression that the flame caused the O2 molecules to somehow disappear out of existence.
Actually, wouldn’t some of the O2 turn into H2O and condense on the surface of the jar?
I assume that carbon atoms from the solid fuel would bond with the O2 to create carbon dioxide. You could be right about the the water vapor although offhand I’m not sure where the hydrogen would come from.
I hope you will stop short of what my father did. We conducted this experiment and it worked perfectly. Being the inquisitive sort I asked papa how we could get the egg back out. He responded (correctly) that we need to reverse the air pressure. He tilted the jar up so that the egg once again became a one-way valve and blew mightily into the jug. Sure enough, the egg blasted out striking him squarely in the face. In addition to having egg on his face (literally and figuratively), he also had the charred remains of the paper we burned to heat the air.
If you have a plastic water bottle, you can show what happens by crumpling the bottle a little and then screwing the cap on, then heating it with a hair dryer. After a few seconds the air in the bottle will heat up and the plastic will pop back into its normal shape. Or, if you blow hot air from the hair dryer into it and then screw the lid on and then dunk it in some ice water (or blow air conditioner air over it, or some other way of cooling it down) the bottle will crumple up as the air inside cools and contracts and outside air pressure presses harder on the bottle.
In fact, with a hair dryer handy, you could turn the egg-in-a-bottle back over (so the egg is sitting inside it) and heat the side of the bottle up, and as long as the egg has a good seal on the inside of the bottle, the egg should pop right back out.
Some oxides created by combustion are heavier than the (usually) metal that was burned, indicating that some of the oxygen has indeed been locked up as part of the calx. In these cases, it could certainly be the case that the pressure was reduced in the jar because some of the oxygen (or chlorine) was “consumed” by the burning.
Yeah, but that’s only if you’re burning a metal. Most versions of this project will use paper, wax, or some other organic material, though, which means that you’re basically burning carbon and hydrogen. The carbon will convert O[sub]2[/sub] molecules to CO[sub]2[/sub], keeping the number of molecules constant, and the hydrogen will convert O[sub]2[/sub] molecules to H[sub]2[/sub]O , increasing the number of molecules. And all of the properties of ideal gases depend only on the number of molecules of gas, not on their type.
It would depend on what you’re burning. The paper mentioned by another poster for example is mostly cellulose, which in turn is mostly C and H with some O - there’s your hydrogen!
Indeed - and in fact most common fuels are part hydrogen; certainly anything in the wood, coal or petroleum line. Charcoal, coke and anthracite are all high-carbon, low-hydrogen fuels but paper or wood, very roughly, will burn to one molecule of water for every one of carbon dioxide (as they have about two hydrogen atoms for every carbon).
Am I crazy for wanting to read the title as ‘Ear in the Jar Experiment’ every time?
I don’t know about “ear”, but I read it as “ego” at one point.
It might be more accurate to say that the total force exerted on the egg by the smaller number of molecules in the jar is overwhelmed by that of a larger number outside the jar.
This is accurate provided you’re willing to agree that you encounter a vacuum when you climb a mountain. I’d say “partial vacuum” or simply “reduced pressure” might be better.
I think everyone else knew that I didn’t mean a total vacuum, but a partial vacuum.