Is it possible for the air makeup in a jar to change?

In class, were doing this thing where You have to burn a candle in side a jar. That jar is inside a pan with a little bit of water in it. The lesson is that fire needs oxygen to burn. I understand that, but were also learning about the atmosphere. The atmosphere is 21% oxygen. What I want to know is if it is possible for the jar to be 21% oxygen one time, and uhhh... i don't know, 15% oxygen the next? My science teacher says it isnt. Theremay not be a signifant difference, but IS there a difference?Is it possible for the air makeup in a jar to change?
When people say oxygen, they usually mean the gas that we breathe, which is really a molecule made up of two oxygen atoms, also called O2 (a molecule is just two or more atoms joined together). Carbon dioxide is CO2 (it's a molecule made of one carbon, and two oxygen atoms). Water is H2O (an oxygen atom with two hydrogen atoms attached). If you were writing it out on paper, the number would be a subscript (a smaller number, slightly lower than the letter), but unfortunately, Y!A doesn't let you type in subscripts.





On to your question, though. The oxygen content in the jar won't change unless you change it somehow. In other words, if it's just sitting there at 21% O2, then it will stay at 21% O2.





If you light the candle, though, things will begin to change. The fire is actually a chemical reaction, where the O2 in the air reacts with the molecules within the candle (the molecules in the candle are made up mainly of carbon, hydrogen, and maybe a little nitrogen) The carbon and O2 shuffle their atoms around, forming CO2, H2O (water vapor) and a few other products. This also releases a lot of heat. Since the Oxygen is consumed in the reaction, if you were to light the candle, the oxygen content in the jar would decrease, and the CO2 and H2O content would increase. The mass inside the jar would stay the same, as would the number of atoms, but as those atoms get shuffled around, the amounts and types of different molecules in the jar will change.Is it possible for the air makeup in a jar to change?
The answer to your question is dependent upon what you consider a difference. In your body, oxygen is absorbed into your blood stream and carbon dioxide is released. So when you exhale you are releasing the carbon dioxide to the air surrounding you. Of course there is a difference although minuscule, in the amount of oxygen in that breath. But it rapidly dissipates due to diffusion of molecules of all the gases kind of mixing together quite easily and the carbon dioxide essentially vanishes into the nitrogen and oxygen which are the dominate gases. Think about it and you will realize that there is little change. Or in the experiment you are talking about, you can introduce a few bubbles of air into the jar and find that the candle can continue burning in an atmosphere that has lower oxygen levels than normal atmosphere. But you won't find any spot nearby with 15% oxygen. So what is significant? But a couple of molecules of oxygen may be missing from right next to your mouth as you breath, but so what.
If you were to put in a chemical binding agent which binds the oxygen, then the level of free oxygen would drop over time to a point.... if the jar is closed off and left alone without any additions then NO. The activation energy of any reactions between the gases present in air is too great for any of them to be happening to a significant level.
Contrary to previous answers and to what your teacher says, yes you can change the concentration of O2 gas in the jar. As you say, air typically contains about 21% O2. If you create a reaction inside the jar that chemically converts O2 into anything else (like CO2 or Fe2O3 or any other oxide or any oxygen containing compound) the concentration of O2 in the jar goes down (where else can O2 come from if the jar is closed?). The amount of O2 remaining after the reaction depends on the details of the reaction. For example, if you know the volume of air in the jar and you know how much of that air is O2 (21%), you can calculate how much Fe (for example) it would take to completely use up the O2 by forming Fe2O3. Then, if you put that much Fe in the jar and it all reacted, there would be zero O2 left in the jar. There are some practical problems with getting exactly the right amount of Fe to just react with the O2 so, if you want to be sure, you would use excess Fe (maybe 2 or 3 x more Fe than you need to just exactly react with the O2). You could use steel wool (basically pure Fe that is finely divided) and, with a little water in the jar and some time, the steel wool will rust i.e., reacting to form Fe2O3 or Fe3O4 or FeO (all stable oxides of Fe). You could also ignite the steel wool inside the jar (it will burn in air as long as there is O2) but you should talk to your teacher for help with this because the burning Fe will get very hot (the tiny Fe wires actually melt and look at the melting point of Fe!). Hope this helps