DEMONSTRATION OF THE BURNING OF A CANDLE Iaidor S. Hirsohhorn Madison High School, Madison, New Jersey
IN THE presentation of the subject of oxidation, it is customary to discuss the phlogiston theory to show wherein it failed, and then to introduce the oxidation theory of burning as developed by Lavoisier. A necessary step in doing this is to prove that the burning of all materials is accompanied by a gain in weight. This is easily shown for such substances as copper, iron, tin,. or magnesium, but not readily demonstrated for such ordinary fuels as wood, coal, oil, or wax. Many high-school texts illustrate an experiment to show that the gases formed by a candle when i t burns weigh more than the burned part of the candle.' The writer has on numerous occasions attempted to carry out the experiment as illustrated with unsatisfactory results. The method involves the use of a glass chimney, filled with sticks of caustic soda, held over a burning candle. The following disadvantages of the method were noted in actual use: (1) the gain in weight is small and difficult to show convincingly with an ordinary laboratory balance; (2) often a loss in weight results due to incomplete absorption of the products; (3) the caustic soda melts and dissolves in the water produced, thus dripping down and often extinguishing the candle; (4) the experiment consumes too much time to make its use for class discussion advisable; (5) great care must be exercised that no drafts blow the gaseous products awav from the absomtion chimnev: (6) . , the formation of soot complicates the usual explanation in beginning chemistry, that hydrocarbon oxygen + water carbon dioxide. The writer suggests the following procedure, which he has used with very satisfactory results. An oxygen tank or generator is attached to an Erlenmeyer flask holding a candle and =da.lime &be as illustrated, ~h~ flask, candle, and soda-lime tube are weighed before the experiment on an ordinary laboratory balance, < ,
+
+
Dry oxygen is passed into the flask holding m e canrue. Aglowiug splint, thrust into the flask, ignites the candle. The flask is quickly restoppered with the soda-lime tube assembly, as illustrated. The candle bums brightly,
SOCIA LIME--
- 2 L A 5 5 WOOL
250-ML.
FLASI(
500-ML. FLASK
producing water and carbon dioxide which are almost completely absorbed in the system. When the burning ceases, the oxygen tube is disconnected, and the flask, products, and soda-lime tube are reweighed. The fob lowing are typical results: Weight of flask, candle, and sada-lime tube before experiment 254.5 g. Weight of flask, products, and soda-lime tube after experiment 255.5 g. Gain in weight 1.0 g.
The advantages this are: it yields a positive gain in weight which can easily be shown on an ordinary balance; (2) the manipulation is simple; (3) the time required is only ten minutes; (4) only water and carbon dioxide are produced; (5) the applica'Mc~~~so~.H~m~~so~,nnmFowzsn,"Chem*tryatwork," tion Of method in respiration calorimeters may Gim and Company, Boston, 1938, p. 89; and other general properly be discussed following the demonstration. chemistry textbooks.
