27 - Fireworks (w/ Sam Hartmann!)
In the United States, the fourth of July is commonly associated with fireworks, but the technology of gunpowder came from much earlier. What is the chemistry of the firework, and what is the historical basis? Let’s learn to be scientifically conversational.
General Learning Concepts
1) Chemistry of the firework
a. Fireworks and energy: As described in episode 6 (batteries), energy is the ability to do work. Upon the detonation of a firework, energy, light, and sound are released.
b. Ingredients of the firework: The metal salts responsible for making different colors are placed inside of clay or dough-like lump. An oxidizing agent and reducing agent allow for a reaction to take place; an oxidizing produces oxygen gas (by decomposition – example: 2 KNO3 à K2O + N2 + 2.5 O2) to burn the reducing agents and excite the atoms of the metals responsible for color. To lift the firework into the air, a highly exothermic (heat releasing) reaction using black powder (a mixture of potassium nitrate, charcoal, and sulfur) allow for lift by trapping heat and gas in the bottom of the firework shell and allowing for lift. There are two fuses: one for the lift charge and one that is time delayed to activate the oxidizing agent. The more oxygen released, the more “spectacular” an explosion, which helped develop using different oxidizing agents.
c. Speed of the reaction: It is not in the best interest of the chemist who has developed a firework to create an explosion; while explosions are part of the experience, the main purpose is to illuminate the sky. The prevent the oxidation reaction from consuming the fuel rapidly, large crystals of chemicals are used (smaller surface area, slower reaction)! The reaction vessels are also not thoroughly mixed.
d. How do we see different colors? When different atoms of different elements are exposed to energy, their electrons can become “excited”. There are different energy levels that those atoms can occupy (Bohr model of the atom, episode 24), and electrons can be promoted (or “excited”) to higher levels with that input energy, but this promotion is tenuous and non-permanent. As the electron decays back to its base, or “normal”, state the energy is released at a specific wavelength (because electrons can be thought of as particles or as waves as discussed in episode 18). This wavelength has a certain color associated with it by definition.
i. Some examples: Copper oxide emits a blue color when ignited, strontium chloride produces red, sodium silicate produces yellow, magnesium and aluminum produce white, and barium chloride produces green.
e. What about the sound? Generally described as a sonic boom because of the rapid expansion of the gases within the reaction; this gaseous expansion happens faster than the speed of sound. Other sounds, like crackling noises, are chemical in nature. For example, when lead contributes to a reaction, the heat allows for vapor that crackles as it expands. If the tube containing the reaction is narrow, the rocket will make a whistling noise. 
2) History of firework (pyrotechnic) development
a. Gunpowder: While not a firework, pyrotechnics like gunpowder were first thought to be invented by alchemists in China looking for the elixir of immortality (possibly in the period 600 – 900 AD). While doubtful they found the elixir, heating potassium nitrate allowed for revelation of explosive properties. Gunpowder was banned to be sold to foreigners and was nationalized in production in China during 1067. Still, the secrets of potassium nitrate made their way to Europe by the mid-1300’s. 
b. Development of the firework: May have happened as early as development of gunpower. Reportedly, the first “firecrackers” were simply bamboo stalks tossed into fire that would explode with a bang due to the natural air pockets. Upon the development of gunpowder, these bamboo shoots could be stuffed for even greater effect. By the 15th century, fireworks were widely used for religious festivals and public entertainment. However, colors are most commonly associated with the 1830’s when Italian inventors added metals to the reaction mixture.
3) Benefits and issues with fireworks
a. Ecological impact: Local air quality is impacted by smoke and particular matter, and a common oxidizer perchlorate can also dissolve in water to contaminate drinking water, lakes, and rivers. There can also be plastics and other bits of fireworks that are not fully detonated that are left behind. Heavy metals responsible for the colorations of fireworks can also be introduced into the environment. Still, the clear answer is that the impact of fireworks are still being studied.
b. Economic cost: In 2014, Americans lit off 175 million pounds of fireworks (valued over $500 million dollars). One of the country’s largest firework displays was calculated to bring in 56 million dollars to its local economy. 
c. Injury: Each year, more than 8,000 people in the United States are injured from personal firework use, with half of those being children.
4) Fun Tidbits
a. Sparkler Heat: Depending on your sparkler, these small fireworks can be anywhere from 1800°F to 3000°F (1000°C - 1600°C). However, due to their low mass, there isn’t much thermal energy to cause damage. Still, operate responsibly.
b. The fourth of July in the USA: John Adams mentioned how he hoped that the anniversary of the United State’s independence would be celebrated with “guns, bonfires, and illuminations”. Some historians believe such a display was supposed to be a moral booster; why cannons and gunfire was removed is beyond me. BONUS FUN FACT: “Today, though fireworks are now a well-established July 4 tradition, they’ve still retained some link to their origins: in 2016, according to the U.S. Census Bureau, $296.2 million worth of fireworks were imported to the U.S. from China.”
5) Solicited Naïve Questions
a. How does one make shapes from the fireworks? Generally, by gluing the clay explosive pieces inside of the firework shell in an organized pattern. Still, this is not a perfect science because the orientation will be uncontrollable once detonation happens. To combat this, one can fire multiple fireworks at once to increase the chances of one “working”.