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Combustion of any fuel requires a gaseous substance. Two of the products are also gaseous. So, when combustion is carried out in a closed container, gases are both used up and produced. Any change in the quantity of gas inside the container would affect the total volume of air inside and therefore the pressure exerted by it. Temperature inside the flask also changes with combustion which would also affect the volume and pressure exerted. The change in the amount of gas is in turn affected by the amount of fuel burnt.


The Discrepant Event

The embedded video below is of the Magical Candle demonstration performed at one of the international schools I taught at. It consists of a small lit candle floating on a dissection tray with a little bit of water. A conical flask is inverted over the candle. Watch the video for the rest.

As you may have noticed, two things happen. The water level inside the conical flask rises and the candle goes out. The question is, why did the two things happen?

You could give a simple explanation for it using the following concepts: combustion, fuel, heat, oxygen, pressure.

But here are some challenging questions about the demonstration.

  1. Why did bubbles of air escape from the mouth of the inverted conical flask as it was being inverted over the candle?
  2. Why did the candle go out and the water rise up the flask? Describe an investigation the results of which could provide support for your answer. Describe the results you would expect.
  3. What factors determine the change in volume of air in the container? Justify your answer quantitatively with theoretical consideration of the reaction taking place, of your knowledge about the composition of air and by making some assumptions about the process. Design an investigation which would essentially test your theory.
  4. How would the results be different if we replaced water with another liquid (say cooking oil)? Why?

Concepts involved

  • combustion, fuel, heat, oxygen, pressure, temperature, volume, gas, carbon dioxide, water, stoichiometry, ideal gas law, moles, number of molecules. (Additional concepts: rate of reaction (combustion),

Your explanation must attempt to explain as much of the observations as possible and support it with quantitative (stoichiometric) data.

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