ark (12/23/82)
It is difficult to get through a high school physics course without seeing the following experiment. A candle is fixed the bottom of a basin, and the basin is filled with water. The candle extends several inches above the water surface. The candle is lighted. A jar is inverted and placed over the candle, with its mouth touching the water. The result: within a few seconds the candle goes out from lack of oxygen and the water rises an inch or two into the jar. The explanation usually given is that the candle has used up the oxygen in the jar and that the water has risen to take its place. This explanation is wrong. Puzzles for the interested reader: (1) prove that this explanation is wrong. (2 - extra credit) supply the correct explanation.
leichter (12/23/82)
Well, the return of an old problem. The explanation isn't incorrect, it's just incomplete. As the candle burns, it converts the oxygen in the jar into carbon dioxide, and some water vapor. The actual change in volume of gas would be complex to calculate, I'd expect - each O2 in the air that gets turned into CO2 is a 1-1 molecule sway; but the O2 to H2O reaction is 1 to 4. HOWEVER, CO2 is quite soluble in water, and the H2O probably condenses out, too; so the net effect is that these gases disappear from the gaseous phase into the water, the amount of gas in the jar decreases, and the water rises. This explanation is still only approximate, of course; the burning candle also gives off a lot of complex organic molecules. There are limits to physics and chemistry, however... What's "right" about the standard explanation is that the water does, indeed, rise to take the place of oxygen that's no longer in the gas; and, in fact, the amount of rise is about what you'd predict based on the percentage or \\\of O2 in air. You can demonstrate that CO2 is going into the water, I believe, by dissolving lime (not the fruit) in it first; it will then turn cloudy as CO2 is added. -- Jerry decvax!yale-comix!leichter leichter@yale