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amount of oxygen in the air

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teach1more:
My daughter is doing a project for an elementary science fair.  She is putting a candle in a pan of water and lighting it.  She then turns a graduated cylinder over the candle and when the candle burns out the oxygen, the water rises into the beaker.  She is trying to measure the amount of oxygen in the air in various places such as inside a house, a green house,  near a lake, in a forest etc.  Her hypothesis is that there would be more oxygen in the forest.  She discovered that there was more oxygen present in a parking lot in town as there was in the greenhouse.  I am not sure why this was true, unless the oxygen in the air from the carbon dioxide emitted  by the cars could be burnt.  Could you explain why this happened?

Mitch:
Was the elevation kept constant?

savoy7:
When a candle is burned, it will release other gases.  

Let's see if I have the setup correct:  Pan of water.  Floating candle that is lit.  Cylinder is turned over and place over the candle.  The opening of the cylinder is under water.  Then candle burns for a few seconds and goes out.

Is this correct?

If this is the setup for the experiment, it would be hard to measure the amount of oxygen.  I hypothesize that the candle heats the air in the cylinder.  These particles have to go somewhere, so they bubble out of the opening of the cylinder.  Then the candle goes out.  The air in the cylinder cools and there is in a sense an empty space that the water then fills.  It fills more if the temperature change is greater between the burning candle and the exterior temp.  Maybe in the parking lot, there was more wind/temp change (the greenhouse & house temperatures are probably warmer and the forest probably has less wind that would reduce convection)  which could account for a larger space to be filled in the cylinder.  

To test my hypothesis, recreate experiment and look for bubbles to come out of the cylinder.  It is important that the initial volume of water in the cylinder is determined for or the same in each attempt.  

Just a suggestion

teach1more:
You have it close.  A birthday candle was stuck in the middle of a pie pan with a dab of modeling clay.  The candle was lit and the cylinder was placed over the candle and pressed down on the bottom of the pan.  We tried it again and I didn't see any bubbles come out of the cylinder.  The elevation and amount of water was kept constant.  The lady at the green house did say that she had just put wood in the stove to warm up the place.  Maybe when she opened the stove door it took some oxygen out of the air?  

savoy7:
Okay.  

One last question; burning a candle does use up oxygen, but releases CO2 (along with other carbon based particles).  How does your setup account for this?  Does the CO2 produced take up some space?  If so, how can one determine the amount of oxygen used?

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