In the Classroom
“Experiment with a Candle” without a Candle Duˇ san Krnel* and Saˇ sa A. Glaˇ zar Faculty of Education, University of Ljubljana, Ljubljana, Slovenia; *
[email protected] The experiment involving a lighted candle in a beaker filled with water and covered by a glass, through which the percentage of oxygen in the air is supposed to be demonstrated, has almost disappeared from schoolbooks. However, it is still used in schools and attempts of revive it appear periodically (1). The explanation of the experiment goes something like this: the candle needs the oxygen for burning, and the used oxygen is replaced by water. Birk and Lawson have described many modified executions of this experiment in detail and clarified a few misconceptions that persist in its explanation (2 ). They calculated that only in the case in which all carbon dioxide is dissolved and all formed water vapor condenses is the volume of gases in the glass reduced by 21%. It is possible to conclude that because of incomplete combustion, some carbon monoxide is also formed. When the candle is burning, part of the wax evaporates; these gaseous hydrocarbons are detectable by smell. However, measurements and experiments prove that the candle flame is extinguished before the oxygen runs out (2, 3 ). Nor does all the carbon dioxide dissolve. Passing the gases into lime water is a simple proof that carbon dioxide remains in the gas mixture above the water. The volume of gases in the glass also depends on the size of the glass and the speed with which the candle is covered. All of these findings and conclusions about the kind of gases in the beaker after the candle flame is extinguished lead to a different explanation of the experiment. The reason for the rising of the water level lies above all in the expansion and contraction of gases due to the warming up and cooling down (2–4 ). These findings are responsible for our idea to carry out “the candle experiment” without a candle in two ways. First, hold a beaker upside down above the flame of the candle or, better still, above the flame of a gas burner to catch hot combustion products (Fig. 1). A good procedure is to wait until the condensed vapor that appears at the beginning has vaporized. Then place the upside-down beaker into a dish filled with water, which will rise slowly in the glass. One might conclude that the volume of gases in the beaker is reduced because of condensation of water vapor and dissolving of carbon dioxide in the water. The second experiment involves heating an upright beaker until the condensed water disappears from the walls but the beaker can still be held in the hand (Fig. 2). Then turn the beaker upside down into a dish filled with water. There is only hot air in the beaker; and so water rising into the beaker indicates that the gases formed during burning are of no importance to the outcome of the experiment. We found that in the first experiment at a gas temperature of 400 K, the volume of gases in a 250-mL beaker was reduced by approximately 20%. In the second experiment, air in the beaker can be heated to approximately 450 K. The
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Figure 1. After catching hot combustion gases in a beaker, place the inverted beaker into a dish of water and watch the water rise in the beaker.
Figure 2. Heat an upright beaker over a flame and then invert it in a dish of water. Again, water will rise in the beaker.
reduction of volume is drastic and approximately as quick as in the original experiment with a candle. The volume in a 250-mL beaker is reduced by approximately 35%. When a candle is burning other gases are formed and the temperature is much lower. Nevertheless, the two experiments described above are good evidence that we have to take into consideration the warming and cooling effects of the gases. Both experiments, in the suggested order, could be done following the “classic” experiment to encourage students to consider alternatives to the usual explanations of replacing oxygen by water or condensation and absorption of water vapor and carbon dioxide. The experiment of a burning candle in a cylinder is simple, interesting, useful, and motivating. Facts that conflict with traditional explanations can teach a great deal about the scientific process. However, other experiments are more appropriate to show that air contains 21% oxygen (5, 6 ). Literature Cited 1. Fang, C. H. J. Chem. Educ. 1998, 75, 58–59. 2. Birk, J. P.; Lawson, A. E. J. Chem. Educ. 1999, 76, 914–916. 3. Torraca, E. In Proceedings of 2nd European Conference on Research in Chemical Education; Bargellini, A.; Todesco, P. E., Eds.; University of Pisa: Pisa, Italy, 1993; pp 207–211. 4. Peckham, G. D. J. Chem. Educ. 1993, 70, 1008–1009. 5. Martins, G. F. J. Chem. Educ. 1987, 64, 809–810. 6. Birk, J. P.; McGrath, L.; Gunter, S. K. J. Chem. Educ. 1981, 58, 804–805.
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
