Fireworks in AP Chem
Posted September 27, 2012
We recently dropped in on Nick Huck's AP Chemistry class where boys were experimenting with electrons, energy and fire.
Garbed in bibs and goggles, the boys were first reminded of safety procedures which include a fire blanket and an emergency shower. After a brief demonstration by Huck, the boys broke into teams , fired up their bunsen burners and got to work simulating the chemical reactions commonly used in the creation of fireworks. The results were instantaneous as students introduced various chemicals to the flames to produce varying colorful effects.
Huck explained the lab: "In an atom, electrons surround the nucleus at specific energy levels. The electrons prefer to be in the lowest energy levels – the ground state. When you put a metal in fire, the fire provides the energy for the electrons to move to a higher energy level – the excited state. The electrons in the excited state fall back to the ground state. As they fall, they give off energy, sometimes in the form of visible light. The difference in energy levels in each atom provides the different colors observed. This is the chemistry that occurs in fireworks.
A Teaching Moment
More than 1,000 years ago, most likely in China, somebody discovered that a mixture of sulfur, charcoal, and saltpeter (potassium nitrate) burned with startling speed and flash. Yes, they had discovered gunpowder, which the Chinese people used it in ceremonies to ward off evil spirits. In the 1200s gunpowder was exported to Europe where, during the Middle Ages, gunpowder-based creations created a few booms and sparkles, mostly in oranges, yellows, and the occasional white-hot.
In the 1800s chemists began using synthesized compounds to make fireworks that, when mixed just so, burned in shades of red, green, even blue. Since then we have moved way past 'the rocket's red glare' stage as the annual Macy's Fireworks display confirms.
Over the past 20 years connoisseurs of fireworks have noticed that colors have gotten progressively more vivid. Even blue (the most difficult color to produce) has evolved from a weak bluish white to something closer to true blue.
These modern fireworks operate on the same principles as fireworks always have. Orangish hues of ancient fireworks are largely produced by black or gray-body radiation--the glow of very hot solid particles. Greens and reds are the spectral emissions of excited gas-phase molecules. In fact, a few metal chlorides, which fluoresce strongly in the visible wavelengths, are the basis for almost all modern fireworks colors:
Barium chloride produces green; strontium chloride produces red; copper chloride produces blue and so on. The typical fireworks mixture consists of, in addition to the fuel and oxidizer, a compound containing one of the metals and a chlorine-donating compound. The mixture is wetted down to bond it together, then cut into chunks known as ‘stars’ --the dots that burst dramatically into the night sky.
In the old days several chemicals were popularly used for fireworks but had the disadvantage of being lethal poisons including paris green (copper acetoarsenite), calomel (mercurous chloride), and realgar (arsenic sulfide). "Oooh, ahhh"… silence. Nowadays fireworks makers rely on compounds such as barium nitrate, strontium carbonate or nitrate, sodium oxalate, and copper carbonatem, and probably live a heck of a lot longer.
Blue has always been a problem for fireworks designers. The copper chloride that produces a blue tone doesn't survive well in a hot flame but good news! A big advance in fireworks colors has arrived in recent decades, with the use of a magnesium-aluminum alloy known as magnalium. Stars made with magnalium burn electric, almost fluorescent, green, red, yellow, and comparatively decent shades of blue and purple. So you weren’t imagining it. Today’s fireworks actually are brighter. On their own, magnesium and aluminum make silvery, sparkly effects and act as a fuel. High heat generated by metal fuels can also increase the intensity of the colored molecular emissions.
Even with magnalium, though, a great blue continues to be fireworks makers' fantasy that they continue to chase like the Holy (Blue) Grail. Luckily for us, we prefer our fireworks in a vibrant shade of green.
Want to know more? Read this Chemical & Engineering News article.