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Pressure Experiment

  1. Nov 10, 2015 #1
    Hi, we did this experiment as an introduction to pressure where you stand a candle on top of a small puddle of water on the bottom of a plate. You then put a glass jar over it and when the candle burns out the water level goes up inside the jar. I found this which shows the experiment if I haven't described it properly:



    We were told that the candle burns the oxgyen in the glass jar and this is why it rises. However after reading I found contradicting sources e.g. http://www.stevespanglerscience.com/lab/experiments/why-does-the-water-rise/. Where it says that this line of thinking is a common misconception. Please could someone explain the correct way? Thanks.
     
  2. jcsd
  3. Nov 10, 2015 #2
    i suspect its because the hot gases are now rapidly cooling thus dropping the pressure inside, the water moves to equalize the pressure on both sides. and hah, if you could do this in horizontal fashion you would get a lot more water to enter.
     
  4. Nov 10, 2015 #3
    What do people make to the accuaracy of the science behind this link:

    http://www.math.harvard.edu/~knill/pedagogy/waterexperiment/

    That there are two effects; one chemical and one physical which balance each other our to start off with hence no water level change when the candle is burning. When the candle burns out there is less air molecules than you started with (as more O2 is consumed than CO2 is released) but the pressure is the same as this is balanced by the increase in temperature. Since the candle has gone out the temperature falls which will decreases the air pressure but not really because the temperature fell so much but because the there are less air molecules than you started with. If you heat up the air and cool it down there would be no pressure change so there must be a loss of air molecules. I definitely did not observe any bubbles coming out from underneath so I don't think air molecules are lost this way but more because of the chemical symbol equation showing less CO2 is released than oxygen is consumed.

    I think it seems to fit better because if the oxygen consumption explanation was the only cause then the water level would gradually increase form the start and it doesn't move at all until the candle goes out.
     
  5. Nov 10, 2015 #4
    Here's what I would hypothesize:

    The air above the candle is hot while the candle is burning. That air is less dense than the surrounding air. When you place the glass jar over it, you are trapping the hot air inside it. The candle burns out (because of lack of oxygen) and the air inside the jar cools. That air becomes more dense and creates a volume of relatively low air pressure inside the jar. The pressure difference sucks the water up.

    An interesting experiment would be to do the experiment again and reduce the amount of space in the jar that is present above the candle. So make it less tall. Maybe also make the jar fatter so as to keep the jar volume constant. The less space above the candle, the less hot air you trap when placing the jar over it, and (I would think) the less air there is to cool and create a zone of low pressure, and the less water to rise.
     
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