Method: Using melted wax adhere a tall candle to the
inside bottom of large pan or dish. Fill the dish about half-way with
water. Light the candle. Invert over the candle a large graduated
cylinder or test tube. Observe what happens and explain why.
-
volume of air decreases as a result of combustion as seen by
rise of water in cylinder.
-
candle flame heats gases inside - when candle flame goes out
gases cool, contracting to a volume smaller than that of air initially
trapped inside cylinder
-
air bubbles are lost as they bubble out of container
-
combustion reaction requires a minimum concentration of
oxygen gas to continue - when oxygen gas level drops below this threshold
the candle goes out with a large fraction of the oxygen gas initially
present remaining
-
gravity necessary for combustion of candle (burning produces
carbon dioxide and water which while hot are less dense than air) - low
density products rise as they are displaced by denser air - if there is no
convection to remove product gases and supply fresh oxygen to flame, the
candle goes out
-
Lavoisier was able to estimate that about 4/5 of air was not
consumed during burning (nitrogen) and that the other fifth was not
destroyed but combined chemically during the burning (oxidation)
