Instructor Information
JCE Classroom Activity: #34
Burning to Learn: An Introduction to Flame Retardants Background This activity demonstrates the effectiveness of flame retardants. Background information about combustion and flame retardants can be found in the work of Kesner and de Vos (1), which led to this Activity. The use of boric acid was suggested by a children’s science experiment written by Robert Krampf (2). Sodium chloride was added as an example of a substance that is not a flame retardant (although flames can be smothered with solid salt).
This Activity can be introduced when discussing combustion reactions or during a unit on practical or everyday chemistry. It could also be used in a safety unit. The Activity introduces the preparation of saturated solutions. If boric acid is used, the greenish flame produced may lead to a discussion of electron quantization and atomic emission.
About the Activity Strips of paper are soaked in solutions and dried in an oven or Be Safe! Because fire is used in this Activity, with a hair dryer. If the humidity is low, the strips will probably dry sufficiently at room temperature for use in less than an hour. They special caution is required. Students should be supervised by the instructor or another adult at all times. can be left overnight to dry. Take care that smoke from burning paper does not set Baking soda and table salt can be obtained from a grocery store. Boric acid is sold in pharmacies. Laundry borax from the grocery off fire alarms or smoke detectors and that all glowing store can be substituted for boric acid, or if desired, both boric acid ash is extinguished. Ash can float through the air, reand borax can be used and the results compared. Saturated solutions sulting in the possibility of fire spreading. Students of the three solids can be prepared by the instructor in advance, if should use metal laboratory or kitchen tongs to hold desired. If students prepare solutions as directed, about a teaspoon paper. of each solid should be sufficient. Students can share the solutions. Excess solution can be poured down the drain. Typical Results. A: paper continues to burn rapidly when removed from the flame; B: wet paper does not burn; C: same as A; D: (baking soda) flames extinguish when paper is removed from the flame; E: (sodium chloride) same as A; F: (boric acid) the paper is charred by a green flame, which then goes out. Results for laundry borax are similar to those for boric acid except that the flame is not green. When heated in a clean, dry test tube, baking soda and boric acid (or borax) produce water, which can be seen as it condenses higher in the test tube. Carbon dioxide is also produced when baking soda is heated. It can be detected by passing the gas through lime water. No reaction occurs when sodium chloride is heated.
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Answers to Questions 1. Water, baking soda, and boric acid retard flames. Water only works as long as the paper is wet. Baking soda reacts when heated, 2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g), to form carbon dioxide and water vapor. Carbon dioxide and water are not combustible and help to block the paper from air, which contains the oxygen needed for burning to continue. When boric acid is heated it also forms water, H3BO3(s) → H2O(g) + HBO2(s). When laundry borax (Na2B4O7ⴢ10H2O) is heated it gives up its waters of hydration. Both boric acid and borax cause charring on the surface of the paper, which helps protect the rest of the paper. In addition, all of these decomposition reactions are endothermic, removing energy from the flame and slowing the combustion process. 2. Flame retardants may make it more difficult for a fire to start or slow its rate of propagation, but they do not make the material completely fireproof. The paper strips will eventually burn if left in the flame. 3. For example: A flame retardant should be a stable material that can be easily incorporated into the fabric without changing its other properties. It should remain active for the lifetime of the object made from the fabric and should not be removed when the fabric is cleaned. It should not be toxic or produce toxic fumes when exposed to fire. 4. See reference 1.
This Activity Sheet may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Integrating the Activity into Your Curriculum
References and Additional Activities 1. Kesner, M.; de Vos, W. J. Chem. Educ. 2001, 78, 41–45. 2. MaximumKids.com—Science Experiment: Flame Retardant: http://www.maximumkids.com/scienceexperiments/ krampfflameretardant.html 3. Committee on Toxicology, Commission on Life Sciences, National Research Council. Toxicological Risks of Selected Flame-Retardant Chemicals; National Academy Press: Washington, DC, 2000; Table of Contents; http://books.nap.edu/ books/0309070473/html/index.html. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 78 No. 3 March 2001 • Journal of Chemical Education
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JCE Classroom Activity: #34
Student Activity
Burning to Learn: An Introduction to Flame Retardants You have probably learned in chemistry class that a fuel, oxygen, and a sufficiently high temperature are required for a fire. You have probably also learned about putting out fires with water and fire extinguishers. Flame retardants, substances added to combustible materials to slow down or hinder burning, are a multi-billion-dollar industry. Such additives in plastics, building materials, furniture upholstery, curtains, and clothing (especially children’s sleepwear) can reduce damage caused by fire and can save lives by giving people time to escape a fire. In this Activity you will test some common substances to see how they change the way paper burns.
Try This You will need: filter paper or coffee filters, scissors, an 8- or 9-in. aluminum pie pan (or a similar container), metal tongs, bottle of water or sand, warm water, pencil, matches or lighter, Bunsen burner or candle, measuring spoon, baking soda (NaHCO3), table salt (NaCl), boric acid (H3BO3) or laundry borax (Na2B4O7ⴢ10H2O), 3 small containers (to hold about 20 mL of solution), spoon or stirring rod, and paper towels. An oven or hair dryer is optional. A fire extinguisher should be available at all times. __1. Cut filter paper or coffee filters into 6 strips about 1 cm wide and 10 cm long. Label the strips A–F with a pencil. __2. Prepare saturated solutions (about 20 mL) of baking soda, sodium chloride, and boric acid. Put about 20 mL of warm water into each of three small labeled containers. Add the appropriate solid to each and stir until no more solid dissolves and a small quantity is left at the bottom of the container. Allow the solutions to cool to room temperature. These solutions can be shared with your classmates. __3. Treat the strips as follows. To dry strips C, D, E, and F use an oven set to about 100 °F, secure them between two paper towels and dry with a hair dryer, or leave them on a paper towel for an hour or longer. A: No treatment. Be Safe! This activity uses fire. Do not work B: Soak in water and test while still wet. alone! Locate emergency exits. Smoke from burning C: Soak in water, then dry. paper may activate smoke detectors or fire alarms. D: Soak in baking soda solution, then dry. Work in a fume hood, a well-ventilated area (near an E: Soak in table salt solution, then dry. open window, for example), or outside. Watch for F: Soak in boric acid solution, then dry. smoldering ashes that can float away and start a fire. __4. Light a Bunsen burner or candle. If you see any, extinguish them at once. Have a fire __5. Hold one end of strip A with metal tongs, and touch the free extinguisher, bottle of water, and/or container of sand end of the strip to the tip of the flame. Remove the paper from available to put out fires. Work over a nonflammable the flame immediately. What happens? After a few moments surface such as metal or concrete. observation, drop the paper strip into an aluminum pie pan and, if necessary, extinguish the flame with water or sand. Record your observations. __6. Repeat step 5 with each of the remaining strips. Strip B should be wet when placed in the flame. All other strips should be dry. Optional: In addition to the items above, you will need 3 clean, dry test tubes. __7. Place a small quantity of baking soda (about the size of a pea) in a clean, dry test tube. Hold the tube with metal tongs and heat over a flame. What happens? Write a chemical reaction to explain your observation. __8. Repeat step 7 with table salt and boric acid (or borax). Use a separate clean, dry test tube for each solid.
Questions __1. Which of the substances tested reduced or prevented burning of the paper? How did they seem to work? (The optional steps above provide additional clues!) __2. Do flame retardants make materials completely fireproof? What would happen if strips B, D, and F were left in the flame for several minutes? __3. Describe desirable characteristics of a flame retardant for treatment of fabrics (drapes, carpet, clothing, etc.). Explain your reasoning. __4. Learn about commercial flame retardants from the WWW or from material provided by your teacher. How do different types of flame retardants work?
Information from the World Wide Web (accessed January 2001) 1. Encarta Encyclopedia. Search for “flame retardant”: http://encarta.msn.com 2. Firehouse.Com News, 4/28/00. Study Examines Flame Retardants: http://www.firehouse.com/news/2000/4/ 28_APflame.html This Activity Sheet may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
