Yukuan- I think you are closer to right than the teacher is. Burning certainly uses up some O2 molecules, but it creates a CO2 molecule for each one. At a fixed temperature and pressure, either type of molecule requires the same volume in the gas. So you could imagine other types of chemical reactions that would change the volume of the gas, but not this particular one.
Why then does the gas shrink? While the flame is burning, the gas is hot, which makes it take up more volume, at a fixed pressure. When the gas cools, it shrinks. There can be some other complications, such as molecules dissolving in the water, where they don’t take up much volume, but the most dramatic effect you see- the water rising after the candle goes out- should be mostly due to simple cooling.
Other experiments, such as the electrolysis of water into oxygen and hydrogen gas, and the burning of those gases to make water, might make better illustrations of how burning uses up something from the atmosphere. Perhaps the oxidation of iron, which makes solid iron oxide compounds, would be another good illustration of reactions using up the oxygen gas in the atmosphere.
(published on 11/10/06)