What NOT to Do for a Grease Fire

When a pan filled with grease is heated to a high temperature during cooking, the vaporized grease can begin to burn. The additional heat energy released by the open flame will cause more and more grease to be vaporized and join in the combustion reaction. You should NEVER throw water on to a grease fire, since this will intensify the reaction. This is due to the fact that when the water comes in contact with the superheated grease, it will immediately vaporize. The liquid water vapor expands approximately 1000 times its volume as a liquid, carrying more grease particles with it. This increases the surface area of the grease, causes the rate of combustion to dramatically increase. The result is a flash of heat and light energy as the grease bursts into flames.

MATERIALS: hot plate, 1 large paraffin candle or block of paraffin wax, old frying pan(not Teflon coated) with lid, matches, metal ring stand base or porcelain hot pad, water squirt bottle, newspaper, baking soda

SET-UP: Prepare the floor of a 3 square meter area by spreading a layer of dampened newspapers. Remove all flammable materials that may be above or immediately next to this floor area. Make sure that your set-up is not directly beneath a fire sprinkler or smoke alarm! When the candle wax ignites, a burst of flames will rise to a height of 2 meters. Since the paraffin is a non-ionic compound, the vapors should not set off a smoke alarm, but the heat may do so. The demonstration can be performed outside on a calm, windless day, if there is not enough clearance indoors.

Position the hot plate at the center of the dampened newspapers. Preheat the frying pan for 3-5 minutes, then add the candle wax. Heat the wax in the pan on the hot plate until it melts completely and begins to vaporize. Then drop three or four UNLIT stick matches into the molten wax. When the wax becomes hot enough, the matches will ignite, causing the wax vapors to begin to burn. Make sure you unplug the hot plate at this time to avoid getting an electric shock. Stand about 3 meters away from the burning wax, and use the squirt bottle to spray water onto it. A burst of hot, burning, gaseous candle wax should erupt from the pan in an upward direction. Any unreacted wax vapor quickly cools, and it will solidify and settle down onto the newspaper. Several bursts of flame can be generated by additional squirts of water into the pan. Be aware that if the wax becomes too hot, the fire may begin to burn continuously. If this happens, simply sprinkle some baking soda into the pan to extinguish the flame (or cover the pan with its lid). Both of these methods will remove the oxygen available for combustion, and smother the flame. Baking soda is most effective because, upon heating, it decomposes to form carbon dioxide as well. Remove the pan from the hot plate and set it onto the base of a metal ring stand(or a fireproof, porcelain hot pad) until it cools off. While the pan is still warm, use damp paper towels to wipe out the wax and baking soda. This demonstration dramatically emphasizes that water should never be thrown onto a grease fire. It also shows how the surface area of a combustable material directly effects the rate of combustion.

Reference: Mark Case, Woodrow Wilson TORCH Institutes, CHEM 6 Team binder, 1995

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