You know it's the Fourth of July when you hear the whistling and booming sounds of fireworks. The U.S. imported $223.6 million worth of fireworks from China in 2011 according to the U.S. Census. Fireworks are one way we celebrate our nation's birth, which began with 13 colonies and a population of 2.5 million, on July 4, 1776. Just as the our population has grown, to an estimated 313.9 million (the nation's estimated population on this July Fourth), so have our fireworks displays.
Early Chinese fireworks - Chinese alchemists searching for an "elixir of immortality" experimented with potassium nitrate. Instead they discovered the explosive properties of the salt. According to legend the Chinese used fireworks to ward off evil spirits on New Year's Day.
Chemistry and colors - The chemicals in the fireworks create the bright hues according to the University of Wisconsin . There are five main compounds that produce fireworks colors.
* Aluminum, magnesium or titanium make silver
* Barium makes green
* Copper makes blue
* Magnesium makes white
* Sodium makes yellow
* Stronium makes red
* Stronium (red) and copper (blue) make purple
How fireworks get off the ground - Combustible black powder, make with potassium nitrate, 15 percent charcoal, and 10 percent sulfur, sends the fireworks high into the sky.
Fireworks shapes are carefully planned - How the fireworks are packed into their shells determines the shapes they create in the sky. Elaborate displays set off more than one set of fireworks at a time, creating layered shapes. Popular fireworks shapes include:
* Chrysanthemum - A flower-like display that is round and symmetrical.
* Palm tree - The shell leaves a trail, which looks like a tree trunk. When the shell opens it looks like palm fronds.
* Salute - Creates a bright flash of light accompanied by a loud booming sound.
* Split comet - "The comets split open to create a golden Milky way."
* Willow - The willow is similar to the chrysanthemum, except the color falls in slow trails
Fireworks and a sonic boom- "Fireworks generate three very noticeable forms of energy: a tremendous release of sound, bright light, and heat. The tremendous booms heard at ground level are the result of the rapid release of energy into the air, causing the air to expand faster than the speed of sound. This produces a shock wave, a sonic boom."
* Salutes made with flash powder or a titanium produce a flash and a bang.
* The whistling sounds are created with tubes shot up into the air.
Leave fireworks to to experts - In 2011, approximately four consumers were killed and 9,600 were injured using professional-grade or homemade firework devices according to the U.S. Consumer Product Safety Commission.