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Fireworks

If you enjoyed a Fourth of July evening pyrotechnic display, or perhaps witnessed a New Years’ Eve event, you’ve witnessed the results of over a thousand years’ worth of research and development into the art of fireworks. The first efforts were produced in China initially by accident as they observed that when saltpeter (potassium nitrate, $\ KNO_3$) was dropped into a charcoal fire, the mixture “popped” and produced an interesting flame color. Later, as a means to surprise their enemies in battle, the earliest “Shock and Awe” campaigns featured a mixture of saltpeter, charcoal and sulfur. The mechanism by which fireworks operate involves heating the proper ratio of these materials (75% $KNO_3$, 15% carbon and 10% sulfur), and generating a chemical reaction to produce nitrogen and carbon dioxide gases. These initial “gung pow” were mainly explosive devices directed into the air, but later new additions brought whistling sound effects and a spectrum of colors to dazzle their opponents. The energy needed to propel the shell and to excite the composite atoms is still provided by a gunpowder formula.

The brilliant colors that produce the oohs and aahs of today’s displays are mainly due to elements like magnesium, which results in a blinding white effect. On an atomic level, the energy imparted by the explosion causes the atom’s electrons to be promoted to a higher energy level. When the atoms relax back to the ground state, a specific amount of energy is released and the color of visible light reveals the frequency of light corresponding to that energy value. The red coloration is due to the presence of lithium or strontium salts such as lithium or strontium carbonate. Sodium salts (usually nitrate) generate a yellow hue and calcium chloride or sulfate result in orange coloration. Barium chloride supplies a green color. The all-American red, white and blue display is difficult to construct due to the complexity of finding a blue colored explosive. Usually copper chloride in a blue-violet hue is substituted. This copper salt’s instability at the high temperatures of the exploding device has caused modern day pyrotechnical researchers to continue the search for a reliable source of blue color.

The shape of the image produced when the shell explodes in the air is a function of how the components are arranged in the shell. When the pyrotechnic device explodes as the resultant gases are produced, the arrangement of the salts in the mortar shell will mirror the pattern produced by the explosion. Dividing the materials into different compartments can also produce a “time – delay” effect, where the display effects occur sequentially.

The sound effects employed as a counterpoint to the visual display are also the result of chemical reactions. Adding bismuth trioxide to the mixture generates “popping” noises, whereas copper salicylate yields a “whistling” sound.

On the next occasion when a fireworks display rises to the sky, you’ll not only enjoy the beautiful visual effects, but have an appreciation for the science that went into the presentation.

Jan 07, 2014

Jul 21, 2014