Resumen de Changes of CO2 Concentration and Heat Illustrate Why the Flame Is Extinguished in the Candle-and-Cylinder Experiment

Ronghui Qu, Sha Sha, Liqun Shen, Yanlin Xiong

• The extinguishment of the candle flame in the well-known candle-and-cylinder experiment has been erroneously viewed as caused by the complete consumption of oxygen, for many reasons. To address this misconception scientifically, a series of experiments are carried out to illustrate the phenomenon from a new point of view. Limewater [Ca(OH)2] placed at different heights in a container was used to qualitatively describe the distribution of CO2 in an enclosed cylinder with one or two burning candles. Temperature sensors, CO2 sensors, and oxygen (O2) sensors were employed to visually and quantitatively compare the content of CO2 at different heights within the container. The resulting temperature curves show that the heat from combustion rose first and then descended soon afterward. Chemical and sensor detection of CO2 in the highest two quadrants of the container indicated that the hot CO2 gas rose initially and accumulated there in the highest part of the container. Then, the hot CO2 gas descended gradually and created a CO2 atmosphere surrounding the flame. As a result, the flame was extinguished by the buildup of CO2 and depletion of oxygen in the gas in the vicinity of the flame. The change of oxygen concentration showed that there was a mixing process between the remaining air and the hot CO2. These results indicate that the extinguishment of the flame does not mean that the oxygen in the container was completely consumed. Knowing this helps students understand that the density of CO2 depends on its temperature.