Burning Questions About a Candle: Continued

Test Your Hypothesis by Doing an Experiment

After you feel like you have made enough observations to give you a pretty good understanding of what is actually happening during the burning process, it is time to do an experiment. An experiment results when you ask a question and then develop a method to answer that question. The experiment itself is actually the process of getting the answer.

So, here is what you need to do:

• First, try to ask a good question. Think about the observations that you made, especially those that you don't really understand, and try to formulate some kind of hypothesis (or good guess) about what is happening.
• Once you have a good question, try to think of ways that you can answer that question. What kind of measurements or observations (either with your own eyes or with other methods) would help you get the answer? Think about the kind of data you'll collect - will your data be qualitative (that is, descriptive) or quantitative (with numerical measurements). What do you have available in the lab that you could use to measure changes that occur?
• Once you have some idea of how to actually do the experiment, go ahead and do it.
• Safety First! Make sure that you consider safety at every step. (Note: NEVER put a thermometer into the flame. The high temperature of the flame will break most thermometers.)
• As you design your experiment, consider how you will present your results. A good experiment is reproducible, and so you'll need to outline your steps (procedure) very specifically. Don't get bogged down in making it perfect the first time around, just make sure that you take clear and specific notes.

Be creative - you should be able to design experiments that you can do with what you can find in the lab or that you can easily acquire with very little effort. For example, holding a piece of glass over the candle as it burns (probably at an angle would work better) can help you answer questions about what is evolved or given off during the combustion process. Think of other simple ways of investigating further what is going on and see what kind of good experiments you can come up with.

References

A Burning Candle: An Experiment in Observation
From the Macrogalleria

Candle Flames
From NASA's Microgravity: A Teacher's Guide with Activities, Secondary Level

Burning Questions about a Candle
written by Lon Mathias, from http://pslc.ws/pslcweb/activity/candle/candle3.htm

Michael Faraday (1791-1867): The Chemical History of A Candle, 1860
From the Internet Modern History Sourcebook

Also, check out these references that discuss classic candle experiments:

• "The Persistence of the Candle-and-Cylinder Misconception," by James P. Birk and Anton E. Lawson, Journal of Chemical Education, 1999, 76, 914-916. This article discusses why a candle under an inverted beaker in a container of water does not indicate the percent of oxygen in the air. The following is an excerpt from this article:
  "In conclusion, in every experiment that purports to measure 21% oxygen in air by a combustion process, combustion is not consuming all the oxygen. Rather, the heating of air results in partial expulsion of air. The flame is extinguished by local depletion of oxygen and buildup of carbon dioxide, not by complete combustion of oxygen in the remaining air. When the flame expires, the apparatus and enclosed air cool back to room temperature, causing a pressure differential that results in a rise in the water level."

• "'Experiment with a Candle' without a Candle," by Dusan Krnel and Sasa A. Glazar, Journal of Chemical Education, 2001, 78, 914.

  This article is a follow-up to the previous article, and describes a way to prove the same point using a Bunsen burner.

Other articles that might be of interest:

• "A new use for the candle and tumbler myth," by Gavin D. Peckham, J. Chem. Educ. ,1993 70 1008.
• "Demonstration of the burning of a candle," by Isidor S. Hirschhorn, J. Chem. Educ. ,1941, 18, 107.
• "Evolution of the candle," by G. Griffin Lewis, J. Chem. Educ. , 1934, 11, 367.