Editor’s Note: This is a guest post by Steve Townsend, director of communications in NC State’s College of Physical and Mathematical Sciences. Just in time for the 4th of July!
As they have for well over 200 years, the skies over cities and towns across the country will explode in celebration this July 4. While there’s nothing like a good fireworks show to celebrate America’s birthday, keep in mind that none of those fancy pyrotechnics are possible without a good dose of science.
In honor of Independence Day 2012, we asked Nathaniel Finney, senior lecturer in NC State’s Department of Chemistry, to give us a little “Chemistry 101” on what gives fireworks their brilliant colors. It turns out, a primary ingredient in fireworks isn’t all that different from a common food additive. Here’s what Finney told us:
“The most common ‘color’ associated with fireworks is the bright white incandescence associated with high-temperature combustion. This white light covers the entire visible spectrum, and is typically the result of burning a reactive metallic element like magnesium or aluminum in the presence of atmospheric oxygen to make a metal oxide and release a lot of heat.
“The additional colors of fireworks arise from heating different metal salts. Metal salt is a general term for a complex of a metal ion. Sodium chloride – a.k.a. table salt – is a common example, but there are many other kinds of metal salts. Most salts do not burn the way a raw metal does but rather give off light when they are heated to high temperature.
“When a metal salt is heated (by the combustion of magnesium metal, for example), the individual metal atoms absorb the heat by moving an electron farther away from the positively charged nucleus of the atom, forming what is called an excited state. The excited state of the metal can release its energy and ‘relax’ its electron most quickly by emitting light instead of heat, which is what leads to colored light being given off by hot metal salts.
“Different metals emit different characteristic colors of light, which are associated with the specific energy of the excited state electron of that metal. At high temperature, sodium salts will generally give off yellow light, while lithium and strontium salts emit red light, calcium salts emit orange light, barium salts emit green light and copper salts emit blue light. The specific metal salt for each color must be chosen carefully – some salts are water-sensitive and some are unstable at higher temperatures, and compositions of such salts have to be formulated specifically for use in fireworks.”
I’m sure you’ll never look at fireworks – or your salt shaker – the same way again.