Saturday, July 15, 2006

How Do Fireworks Work?

You probably saw, or at least heard, fireworks earlier this month for July 4th celebrations. The idea that explosions cause loud sounds along with bright lights is commonplace. You've seen it with lightning, sparklers, and a host of other things. But how do firecrackers work? What gives them their beautiful colors and carefully-arranged patterns in the sky?

Fireworks have been around for centuries. They were invented in China, but are now enjoyed all over the world. A firework is basically a timed explosion of a precise chemical mixture in order to produce loud noises and colored lights after the firework has shot up into the sky.

Each firework is composed of two cylinders: a small lifting charge cylinder on the bottom and a large shell on top. The lifting charge is the first source of fuel for the firecracker. When lit, it explodes in order to propel the shell upwards like a rocket.

The shell consists of four main parts. The container (made of paper and strings) wraps around the rest of the shell to keep everything in place. The actual color-producing chemicals (also known as color-emitters) are bundled up into small stars which can be arranged so that the individual bursts of light will form a neat flower-like pattern in the sky. The bursting charge is the fuel for the shell and it is located at the core of the cylinder so that the stars will explode outwards.

The last part of the shell is a fuse which carries the spark from the lifting charge cylinder to the stars in the shell. Since it takes time for the spark to travel, the fuse allows engineers to control the length of time between when the firecracker takes off and when it will explode. Increasing the length of the fuse will delay the explosion for a longer amount of time so that the fireworks will burst even higher in the sky.

Until the production of chlorates (pronounced KLOR-ates) in the 1800s, fireworks only came in yellow and orange varieties since those are the only colors (besides grey) that resulted from burning the firework shells. The addition of chlorates allowed chemists to produce red and green explosions. Safe chemical mixtures that produce blue and purple flames were not developed until the 1900s.

Obtaining pure bright colors is extremely difficult when dealing with fireworks. Many of the color-emitters are so fragile that they would decompose (break down into other chemicals) if stored in the shell of a firework. To solve this problem, chemists devised mixtures of other chemicals that will react to form the desired colored compounds once the firecracker is ignited.

Another dilemma is that, during the explosion, many of the byproducts (unwanted chemicals formed in the reaction) are gray or yellow so these compounds can make the color of the flames look washed-out.

For more information on fireworks, and an interactive demonstration, visit on the web.

Little Lion Experiment:

To see how chemical reactions can produce colors that you might not expect, try this experiment (it's best to have an adult around for the first part). The chemical reactions used in fireworks are very dangerous, so we will be using other chemicals to illustrate the point [do not try to make your own fireworks!].

Grate about half a head of red (purple) cabbage (be careful not to cut your fingers). Place the grated cabbage in a pot with enough water to cover the cabbage, and boil it for 20-30 minutes. The water should be dark blue or purple at this point (if it's not, then boil it for a few minutes more). Then, pour the liquid (careful - it's hot!) through a strainer into a bowl. Throw out the cabbage pieces in the strainer. Let the purple liquid cool.

Pour a few tablespoons of white vinegar or lemon juice into a small cup. In a different cup, mix some detergent or baking soda in water. Then add a few drops of your cabbage juice to each cup. See what happens. Are the solutions the colors you expected?