[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?

[hmx9123]

Don't worry too terribly much about how much CO₂ is produced.  As long as you burn the same type of candles and use a new one every time, you'll wind up with a lower degree of variabiliy.  Also, do two or three experiments and average your findings--it will eliminate possible outliers.

When running this experiment using water, you need to keep in mind the fact that water has a vapor pressure, too, which is NOT insignificantly small.  You need to measure the temperature of the water, because the warmer it is, the higher the vapor pressure of the water.  My guess is that the temperature in the greenhouse (and thus probably the water) was significantly higher than the parking lot.

In addition, make sure that you are careful to keep the inverted cylinder at the same height from the surface of the water each time.  Either sink the mouth to the bottom, or try and keep it just below the surface of the water by affixing it somehow to the container.  Although it shouldn't make too much difference, fill the container to the same level with water from the same source each time to eliminate more variables.

When measuring the level of the cylinder, someone else mentioned the cooling of the gasses inside the cylinder.  Because of this, you need to measure the level of the water inside the cylinder at the same time after the candle goes out each time.  Keep in mind that in a windy parking lot heat is dissapated faster than in a hot still greenhouse.  Take a reading at say, 5 seconds after the flame goes out.  Make sure your eye is reading from the same level each time and that you're reading the meniscus of the water, not some random part of the water level each time.

In addition, you will have to monitor barometric pressure as well; signifacant atmospheric pressure changes may have an effect both on water vapor pressure and the amount of air you're trapping in your cylinder.  This will be a minor contributor, as the atmospheric pressure won't contribute as much as some of these other factors.  If you want to check it out, run three replicates at the same place on two or three different days to help eliminate random error.

I did an experiment with butane similar to this one time, and it was a huge pain to get all these factors nailed down--you could change the level of water inside the cylinder by a huge amount just by the depth of it being inconstant.  It is theoretically easy, and easy to get some crude answers from, but the difference of oxygen you're looking for is going to be small and may be within the error limits of the experiment.