JCE Classroom Activities are edited by Erica K. Jacobsen
Instructor Information
JCE Classroom Activity: #95
A Candle in the Wind
Robert J. Eierman Dept. of Chemistry, University of Wisconsin–Eau Claire, Eau Claire, WI 54701;
[email protected] In this Activity, students investigate physical and chemical changes that occur in a candle (1–3) to learn how a candle functions and how you can blow it out. This Activity is based on a series of lectures presented by Michael Faraday in the 1850s (1). Candles are wax (e.g. paraffin, which consists of straight chain and branched hydrocarbons with ~20 carbon atoms per molecule) with a cloth wick running up through the center. When the wick is first lit, the cloth chars, releasing heat, which melts wax. Liquid wax moves up the wick by capillary action and vaporizes when it reaches the flame. Gas-phase wax diffuses out from the wick and encounters oxygen from the air. Wax burns (reacts) with the oxygen, releasing carbon dioxide, water, other small carbon-containing molecules, and heat. Released heat melts additional wax, which continues the cycle. Students typically believe the wick is burning and not that invisible gas-phase wax is the fuel. In the blue, oxygen-rich zone at the bottom of the flame, combustion of the wax is complete and that zone is very hot. In the top of the flame, a hot, gaseous soup of small molecules gives off light as molecular emission that appears yellow. Only the yellow part of the flame absorbs light and casts a shadow; note the pale shadow in the flash photo (Figure 1). A disturbed flame produces black soot (solid carbon, not completely oxidized). When you blow a candle out, the flow of the gas-phase wax (fuel) to the combustion zone is disrupted and the hot gases (heat) are dispersed, extinguishing the flame. Afterward, gaseous wax emerges from the hot wick for several seconds. You can see and smell the vaporized, then condensed, wax that appears as white smoke.
Integrating the Activity into Your Curriculum Candles are familiar, yet inherently fascinating devices. A huge range of chemical principles is at play. Three phases of matter and the processes of melting and vaporization are present. Gas-phase oxidation/reduction chemistry occurs, as does complete and incomplete combustion (CO2, CO, and C(s) products). Several modes of transport (capillary action, diffusion, buoyancy) are present. Activation energy (a match) and multiple thermodynamic principles are on display. The Faraday lectures themselves are an interesting read and contribute an historical note (1). Using lit candles presents risks associated with open flames, such as burns, lighting other things on fire, and scattering hot wax. A container of water should be within reach of every candle explorer. Demonstrate proper procedure to students beforehand for the scorched card procedure in step 5 and warn them of the following hazard: the card may smolder, then suddenly burst into flame if tipped vertically. Remind students to follow only the listed procedures. Candles should be free-standing or in a stable holder. Short candles can be anchored with clay in an upside-down jar lid. Tea lights are inexpensive and very stable. Candles with a prominent flame that produce a thick white smoke when put out are useful for the jumping flame trick.
perforated
Answers to Questions
The top of the flame emits yellow light and absorbs light, casting a shadow.
photo by Rick Mickelson, UW–Eau Claire
About the Activity
1. The undisturbed flame produces no visible “smoke”, but is giving off invisible carbon dioxide and water vapor. The disturbed flame produces sooty, black smoke, made of solid carbon. 2. The smoke from the unlit candle is white and smells like wax. The smoke consists of vaporized, then condensed, wax particles that come from the wick (until it cools). 3. Gas-phase wax is the fuel. After the flame is blown out, wax briefly evaporates from the wick. It then condenses, appearing as white smoke. A lit splint ignites the wax particles and the flame burns down the smoke trail to reignite the wick. 4. C21H44 + 32 O2 → 21 CO2 + 22 H2O 5. It is donut or ring-shaped. The hottest part is a ring around the wick. Gas-phase wax diffuses outward from the wick where it meets oxygen diffusing inward. The combustion zone where they meet is a circle that produces the scorch mark. 6. The wick transports wax to the flame. Liquid wax climbs the wick due to capillary action. The wax evaporates in the hot flame and moves out into the volume surrounding the wick where it burns. 7. The substances are primarily in the gas phase. 8. The breeze disrupts flow of the reactant gases (wax and oxygen) so they stop reacting. This stops the heat release and extinguishes the flame. 9. See the Background section.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
References, Additional Related Activities, and Demonstrations
1. Faraday, Michael. Faraday’s Chemical History of a Candle; Chicago Review Press: Chicago, IL, 1988. [Also see e-book link at http:// www.gutenberg.org/etext/14474 (accessed Jan 2008)]. 2. Walker, Mark; Gröger, Martin; Schlüter, Kirsten; Mosler, Bernd. A Bright Spark: Open Teaching of Science Using Faraday’s Lectures on Candles. J. Chem. Educ. 2008, 85, 59–62. 3. Rohrig, Brian. The Captivating Chemistry of Candles. ChemMatters 2007, 25 (4), 4–7. 4. A similar analysis of a gas burner flame appears in Chemistry Comes Alive!, Vol. 6 (see Jacobsen, J. J.; Moore, J. W.; Zimmerman, J. F.; Browne, L. M. J. Chem. Educ. 2002, 79, 1381 or JCE Web Software, http://www.jce.divched.org/JCESoft/jcesoftsubscriber.html). Supporting JCE Online Material at http://www.jce.divched.org/Journal/Issues/2008/Apr/abs528A.html
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 4 April 2008 • Journal of Chemical Education
528A
JCE Classroom Activity: #95
Student Activity
A Candle in the Wind photos by Rick Mickelson, UW–Eau Claire
“And it seems to me you lived your life like a candle in the wind. . .” —Sir Elton John Candles are remarkable devices that are used during campouts, in religious ceremonies, and during romantic dining. However, candles, as Elton John points out, have a problem; they are unstable in the wind: they flicker and are easily blown out. What happens when you blow out a candle? Where does the flame go? Like any combustion reaction, a candle flame needs three things: fuel, air (oxygen), and heat energy. In this Activity, you will investigate the secrets of how a candle balances those three necessities to create its beautiful light and how the flame can be extinguished by simply blowing on it.
Try This You will need: candle that is free-standing or in a stable holder (tea lights, thick birthday candles, or tapers work well), several matches and wood splints, two to three 3 × 5 in. index cards, and a container of water. As you perform each part of the investigation, make careful observations and record them. Look at the Questions section at the bottom of the page beforehand to guide your observations. __1. Briefly examine an unlit candle. What do the wax and wick look like? Light the candle and observe it for at least one minute. What do the wax and wick look like now? Describe the colors, shapes, and movements of the different parts of the flame. Is there any odor? __2. Hold a 3 × 5 in. index card horizontally and move it slowly through the flame. How does the flame change in appearance? Describe the appearance of the card’s bottom surface. __3. Blow out the candle and observe any changes that occur in the next 15–20 seconds. How do the wick and wax now appear? Is there any odor? What is given off by the candle? __4. Relight the candle and let it burn for 30 seconds. Light a wood splint in the flame. Hold the lit splint in your hand, blow out the candle, and immediately put the lit splint in the white smoke about 1 in. above the wick. What happens? Why is this called the “jumping flame” trick? __5. Relight the candle if necessary. For a few seconds hold a 3 × 5 in. index card horizontally in the lit flame just above the wick and REMOVE CARD IMMEDIATELY when a scorch mark appears ON THE TOP of the card. BE CAREFUL! If the card ignites, quickly put it in the container of water. Describe the appearance of the top and bottom of the card. __6. Observe the lit candle for a few moments. In which direction do the materials and the flame’s heat energy flow? __7. Blow gently on the flame a few times without blowing it out. Then blow it out.
Questions
Be Safe! Because fire is used in this Activity, special caution is required. You must be supervised by your instructor or another adult at all times. When you are working with a candle, have a container of water to extinguish flames. Take special care with the scorched index card activity; follow your instructor’s directions. ONLY described procedures should be carried out, exactly as stated.
1. What color is the smoke from the undisturbed flame? What color is the smoke from the flame when disturbed by the card in step 2? What chemical substances do you think might be in the smoke under each of these two different conditions? 2. What color is the smoke from the blown out candle in step 3? What does it smell like? What is the substance in the smoke from the unlit candle and where does it come from? 3. What fuel burns in the flame? How does this relate to how the flame was able to “jump” in step 4? 4. Using C21H44 as the formula for paraffin wax, write a chemical equation for the candle combustion reaction. (Hint: Combustion is reaction with oxygen and combustion products are common oxides of carbon and hydrogen.) 5. What shape is the scorch mark in step 5? Based on this shape, where is the flame the hottest? Why? 6. What is the purpose of the wick? 7. What phase(s)—gas, liquid, or solid—are the substances in the flame? 8. What aspects of the burning process are disrupted by blowing on the flame? How does this “put out” the flame? 9. Draw a diagram of the burning candle and describe how it works using all you have observed and learned.
Information from the World Wide Web (accessed Feb 2008) National Candle Association. http://www.candles.org/index.html How to put out a candle without blowing it. http://www.videojug.com/film/how-to-put-out-a-candle-without-blowing-it Images of candle flames in microgravity. http://nix.nasa.gov/, then search “candle microgravity” and go to image range 31–40 This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
