Communication pubs.acs.org/jchemeduc
Creating and Experimenting with Fire Gel, an Inexpensive and Readily Prepared Insulating Material Thomas S. Kuntzleman,* Dakota J. Mork, Levi D. Norris, and Christopher D. Maniére-Spencer Department of Chemistry, Spring Arbor University, Spring Arbor, Michigan 49283, United States ABSTRACT: A method is described to make Fire Gel, an insulating material that consists of water and a superabsorbent polymer. Fire Gel can be used to demonstrate how stunt persons protect themselves from the flame of a fire. A comparison of this Fire Gel demonstration with previously reported flame protection demonstrations allows for instructive discussion. Fire Gel is a useful, easily produced, and inexpensive alternative to the gel described in JCE Classroom Activity #107.
KEYWORDS: General Public, Elementary/Middle School Science, High School/Introductory Chemistry, Public Understanding/Outreach, Hands-On Learning/Manipulatives, Calorimetry/Thermochemistry, Conductivity, Hydrogen Bonding
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INTRODUCTION JCE Classroom Activity #107 vividly illustrates how stunt persons use chemistry to protect themselves when being set ablaze while filming action movie scenes.1 To perform such a stunt, the area on a stunt person to be “burned” is coated with a layer of gel. Fuel is subsequently added on top of the gel, and the fuel is ignited when the scene is ready to be filmed. The gel acts as an insulating barrier between the burning fuel and the body of the stunt person, providing protection from injury and burns. The gels used in these stunts are composed of greater than 99% water, an excellent insulator with a specific heat capacity of 4.18 J g−1 °C−1. Thus, a great deal of energy is required to raise the temperature of water. This property of water is due to strong hydrogen bonds that exist between water molecules. When energy is absorbed by liquid water, molecular motion increases, causing an increase in temperature. However, in order to allow for increased molecular motion, hydrogen bonds between water molecules must be broken. The extra energy required to disrupt hydrogen bonds in water accounts for the larger than normal heat capacity of water. In JCE Classroom Activity #107, a demonstration is described in which a hot dog is protected from the flame of burning isopropyl alcohol by a gel containing water and a superabsorbent polymer (SAP). Unfortunately, SAP-containing gels used for the purpose of fire protection are expensive and often difficult to acquire.2 For example, the commercially available SAP-containing gel described in JCE Classroom Activity #107 costs about $20 per quart. Because we have performed this demonstration numerous times, we have found this cost to be somewhat excessive. As a result, we have devised our own formula for an inexpensive SAP-containing gel that we call “Fire Gel”. Because Fire Gel can be made quickly from simple materials, we have extended JCE Classroom Activity #107 to demonstrate to students how Fire Gel is made. This © XXXX American Chemical Society and Division of Chemical Education, Inc.
extension allows students to see the water-absorbing properties of the SAP in addition to the insulating properties of water in Fire Gel. In this communication, we describe how to make Fire Gel using easily obtained materials. We also report on the effectiveness of Fire Gel in protecting hot dogs from an alcohol flame, and compare the Fire Gel demonstration to other chemistry demonstrations that employ the insulating properties of water to protect objects from flame.
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HAZARDS Great care must be taken when lighting objects on fire with alcohol. This activity must be conducted under the supervision of an experienced, adult chemist. All burning material must be kept inside the aluminum pie pan. The pie tin becomes quite warm during the demonstration; at the conclusion of the demonstration be certain not to touch the pan until it has cooled to room temperature. While this activity demonstrates the chemistry behind how stunt persons safely light themselves on fire, students must be strongly cautioned that they should never attempt such a stunt.
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GEL RECIPE Performing this procedure as a demonstration is useful to show the water absorbing properties of sodium polyacrylate (the SAP found in baby diapers3−5), because many students find it remarkable that a very small amount of sodium polyacrylate can thicken a very large amount of water to form a gel. The materials required to make Fire Gel are powdered sodium polyacrylate,6 water, and a blender. Water is placed in the blender container, and the blender is turned on. While the blender is running, the lid of the blender container is removed, and powdered sodium polyacrylate7 is slowly sprinkled into
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dx.doi.org/10.1021/ed3006506 | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 1. Setup of thermocouple, pie tin, isopropyl alcohol, and hot dog without (left) and with (right) Fire Gel for burn test.
(Figure 3). Energy from the burning alcohol is absorbed by the water, increasing its temperature and even causing it to
water, forming a liquid−gel mixture. The sodium polyacrylate should continue to be sprinkled into the gel mixture until a thick, sticky gel is formed. Fire Gel can be stored for several months in a refrigerator until needed.
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INSULATING EFFECTIVENESS OF THE GEL We have demonstrated to students the effectiveness of the insulating properties of Fire Gel. To do so, a hole is punctured into an aluminum pie tin with a thermocouple,8 which is allowed to rest on the bottom of the pie tin. A hot dog is laid alongside the thermocouple (Figure 1), which is coupled to a data acquisition system connected to a computer for display to a large audience. Next, 5 mL of isopropyl alcohol is poured onto the hot dog. When the thermocouple reaches a stable temperature, the alcohol is ignited and the temperature is recorded for 2 min. In the absence of Fire Gel, the thermocouple registers a large temperature increase (Figure 2, solid trace) and the hot dog is significantly charred (Figure 1,
Figure 3. Temperature recorded by a thermocouple surrounded by a 15 cm × 20 cm towel soaked in a 50:50 mixture of alcohol and water and ignited at about 3 s.
vaporize. The towel itself does not ignite, because its temperature remains at the boiling point of water until the water in the towel completely vaporizes. If the towel is soaked in alcohol only and ignited, the towel burns and chars significantly. In the “boiling water in a paper cup”10 demonstration, a paper cup is filled with water and directly heated with a Bunsen burner flame from below. Energy from the flame conducts through the paper cup and into the water, causing the water to warm up and even to boil (Figure 4, dashed trace). However, the cup conducts the flame energy into Figure 2. Effect of Fire Gel as an insulating layer. Solid trace, no Fire Gel between the thermocouple and burning alcohol; broken trace, Fire Gel between the thermocouple and burning alcohol. The alcohol is ignited at about 15 s.
left). To show the insulating effects of Fire Gel, the experiment is repeated, although the hot dog and thermocouple are coated with a layer of Fire Gel before the alcohol is added and ignited. In this case, a small temperature (
