MadSci Network: Chemistry
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Re: Burning Candle Experiment

Date: Mon Apr 27 21:46:12 1998
Posted By: Robert L. Judge, Faculty, Chemistry/Science Department, Holy Cross High School
Area of science: Chemistry
ID: 893672261.Ch
Message:

Dear Mr. Hunt,

This is a great example of a misconception regarding this experiment. As you know, matter is conserved in a chemical reaction (such as the burning of a candle) and so the oxygen could not be consummed. It must be thought to be rather converted to something else. In the case of combustion reactions, the products are CO2 and H2O. Therefore if the oxygen were used up, it would be replaced by the products of the combustion. There is one other products that exists in this reaction, and that is heat and light. In the case of the heat, the gas molecules in the bottle are heated. As heat is put into the system, the molecules gain kinetic energy and therefore for the pressure inside to remain the same as it was before the introduction of the heat, there are less molecules of gas required. (See the ideal gas Law PV=nRT). Because less molecules of gas are required when the gases are heated, what happens when the gases are cooled? When the gases are cooled, they are loosing kinetic energy and therfore occupy less volume. For the pressure in the bottle to become equalized, either the bottle will collapse or will have water pushed into it by the outside pressure which is greater. A good activity to demonstrate this is to see how high up in the tube or bottle the water will rise. You will see that the more heat you can put into the system, the higher the water will rise. Other activities are collapsing coke cans, sucking a hardboiled egg into a bottle, collapsing a metal can etc. For more, see information concerning the gas laws -- Kinetic theory, Gay-Lussac's, Boyle's and the ideal gas Law.

Robert L. Judge, Chemistry Department, Holy Cross High, New Orleans


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