648 CREATIVE CHEMISTRY ammonium phosphate, or some other salt which partially fireproofs them so that the wood ceases to glow as soon as the flame is extinquished. Modern "strike anywhere" matches are tipped with a mixture of phosphorus sulfide, P4S3, and some oxidizing material such as potassium chlorate, KCIO3, powdered glass or some other material to increase the friction, and glue to bind the ingredients to the match. The wood The Principle of the Match Is Employed in Fireworks. The essential principle employed in the manufacture of matches and fireworks is to use a mixture of a readily oxidizable material and a powerful oxidizing agent, the components of which will react with each other as soon as the kindling temperature is reached. The reaction is usually a specialized kind of combustion in which the oxygen is furnished by a compound which readily gives it up on heating. The old black gunpowder was a mixture of powdered charcoal (14 per cent), sulfur (11 per cent), and potassium nitrate (75 per cent). is usually impregnated with paraffin to render it more readily inflammable. In this case potassium nitrate, KNO3, furnishes the oxygen for the combustion of the carbon and sulfur to form a mixture of gases. Gunpowder is still used for mining operations and in pyrotechnic displays, but it has been replaced for use in guns by more powerful smokeless powders. Firecrackers are made by wrapping gunpowder in paper. Flash crackers use a mixture of magnesium and some oxidizing agent such as potassium chlorate instead of gunpowder. Colored lights are produced by mixing various substances such as strontium chlorate with mixtures similar to gunpowder. Strontium salts produce red fire; barium, green; sodium, yellow; magnesium and aluminum, white; and Paris green blue. Magnesium is used in warfare for incendiaries and flares because, as it burns, its temperature is raised to the boiling-point, producing magnesium vapor which burns in air with a very hot flame of great brilliance. Magnesium is used in incendiary bombs partly because of its low density. It is ignited with thermite. Once ignited, burning magnesium, instead of being extinguished by water, reacts with water to produce hydrogen which burns with even greater intensity than the magnesium itself. Magnesium bombs are extinguished by covering them with sand or asbestos blankets to shut off the oxygen supply. Thermite is a mixture of iron oxide and aluminum, which, when ignited by a primer of aluminum or magnesium powder mixed with some oxidizing agent such as barium peroxide, Ba02, burns furiously.
COMBUSTION 649 Huge thermite incendiary bombs were used in World War II very effectively. Fire Extinguishers Employ Completely Oxidized Carbon Compounds. Water extinguishes fires by lowering the temperature of the burning object below its kindling point and by shutting off the oxygen supply. The steam formed may be a factor also. The various types of more efficient fire extinguishers are based on the principle of smothering the fire by preventing an access of air (oxygen), just as a blanket wrapped around a person whose clothes are on fire will smother the fiame. An excellent example of this smothering action is the use of foam to extinguish fires in liquid fuels. The majority of fire extinguishers use carbon dioxide for one reason or another. One type of fire extinguisher, called the soda-acid type, consists of a fairly large cylinder containing a solution of sodium bicarbonate and a bottle of sulfuric acid. The acid pours into the baking soda solution when the cylinder is inverted, thus generating carbon dioxide. The carbon dioxide liberated in the above reaction produces a pressure that causes relatively large amounts of the carbon dioxide to dissolve in the solution and forces the resulting solution out through a nozzle. In this case, the water serves to extinguish the fire, but the carbon dioxide has little if any value except as an expellant. In guisher. (Courtesy of the the foam American-La type of extinguisher, a licorice extract France and Foamite Industries, Inc.) is added to the carbonate solution to produce a foam, and alum (aluminum sulfate, Al2(S04)3) replaces the sulfuric acid of the soda-acid type. The alum hydrolyzes to produce sulfuric acid in this case. The foam issues from the nozzle, each bubble being filled with carbon dioxide. Some very efficient fire extinguishers use liquid carbon dioxide which produces carbon dioxide snow as it is sprayed on a fire and serves to smother (not cool) the fire by diluting the inflammable mixture of air and fuel. In another type of fire extinguisher finely divided particles of magnesium carbonate, MgCOs, or sodium bicarbonate, NaHCOs, are forced onto the fire with the pressure produced by liquid carbon dioxide or compressed nitrogen. Fig. 283. Section of a soda-acid fire extin-