July 2004

Celebrating important events with fireworks is a very old tradition that probably began in China, more than one thousand years ago. In the U.S., fireworks and the American flag are the icons of Independence Day--the Fourth of July. On that day, Americans traditionally end a day of parades and cook-outs by settling on lawn chairs and blankets to admire an annual spectacle in the sky--a fireworks show.


Chemistry of a Celebration

What chemistry is responsible for the awesome bursts of stars and cascading streams of color in fireworks? The formula for fireworks hasn't changed fundamentally in many years. Fireworks consist of an "oxidizer," a compound that supplies oxygen; a fuel, such as charcoal, and other chemicals that serve as binders or create color and spark. Black powder--a mixture of potassium nitrate, charcoal and sulfur--was used for fireworks before it was adapted as gunpowder. Eventually, the chlorine compound potassium chlorate (KClO3) took the place of potassium nitrate as oxygen-supplier, but it too has been replaced--by a more stable oxidizer--potassium perchlorate (KClO4).

Not only does chlorine chemistry provide the chief oxidizer used in modern fireworks, but a few metal chlorides are responsible for the dazzling colors that flash against the night sky.



Choose Your Color

Before the 1800s, fireworks consisted of loud explosions and a few sparkles, made by bits of metals such as iron, copper or zinc. Colors were limited mostly to shades of yellow and orange. Fireworks became spectacularly colorful when certain new laboratory-made color-producing compounds were added to the chemical mixture. Today most colors are produced by metal chlorides. The table below is a "menu" of chloride color possibilities:

    

More Brilliant Than Ever

Pyrotechnics is the science and art of making fireworks

The chloride compounds in the table above present one disadvantage to the fireworks professional--they are very hygroscopic. That means they absorb moisture easily, becoming damp and less effective as color-producers. The latest advance in fireworks science solves this problem. Instead of producing color from the glow of very hot solid particles of metal chlorides, the new technique yields color by energizing these compounds as they are produced as gases during the burning process in the fireworks. That seemingly small difference makes the colors brighter, according to John A. Conkling, technical director of the American Pyrotechnics Association.

Exciting Electrons Produce
Fireworks Colors!

Does this sound like science fiction? Fireworks and exciting electrons? Well, electrons of atoms really do get excited-when they absorb energy. And when they release that energy, they may emit flashes of colorful light.

You may know that electrons are tiny negatively charged particles that surround and "orbit" an atom's nucleus (see the diagram below). As it turns out, electrons have different energy levels, from the lowest, most stable level, to higher, excited, but unstable levels. An electron may absorb energy (for example, from the chemical reactions in fireworks) and "jump up" to a higher energy level. But because this level is unstable, the electron will soon "fall back" to the low energy level. As it does, it may emit its absorbed energy in the form of a burst of colored light, as shown below:


An atom consisting of a nucleus (red sphere in the
center) and "orbiting" electrons (pink spheres).

Reference:

Wilson, E. (July 2, 2001). "What's that stuff? Fireworks." Science and Technology, Vol. 79, No. 27.

For a list of previous "Chlorine Compound of the Month" features, click here.

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