Boom! Pop! Sizzle! When their gazes were fixed on the streams of red, yellow, green and blue that fell lazily from the sky, the crowds below were astonished by this fantastic scene. It was Fourth of July, as known as Independence Day, which is one of the most essential national holidays in America. Thousands of people gathered from all over the country to watch and be amazed by the wonder of fireworks. Fireworks have been around for centuries and play a main role in celebrating special holidays around the world.
Believe it or not, the reason why fireworks have amazing visual effects because there are several specific and timely chemical reactions behind the fireworks. These reactions require different kinds of components such as oxidizers, reducers, varying amount of metal chlorides and so on, which create the incredible effects we see, as a result, firework. Even though firework is pretty, it still brings a lot of air problems for us.
It is universally acknowledged that firework is one of the most fantastic inventions that people invented.
Throughout history, firework has existed for a long time. Many historians believe that the first natural firecracker was originally developed around the second century B.C. in Liuyang, China. At that time, Chinese people threw bamboo segments onto open fire, would burst with a bang because of the overheating air in the hollowed-out bamboo. People believed that the sound which firecrackers made would drive evil spirit away. Sometime between 600-900 A.D., an alchemist mixed saltpeter (potassium nitrate), sulfur and charcoal accidently and set it on the fire-when bang- created the black powder, which was the first gunpowder.
He added this mixture into bamboo segments or rolled it up with paper tubes, which formed the first man-made fireworks. After that, in 1830, Italian pyrotechnicians added color to fireworks with chlorinated powder and metallic salts.
The chemistry of firework is based on the reactions of combustion. Inside the firework, there are five vital components: oxidizing agents, reducing agents, fuel, coloring agent, and binder. Oxidizing agents are usually oxygen-rich ionic solids that will decompose at medium or high temperature to produce enough oxygen in order to support the mixture inside firework to burn. In addition, these oxidizers should be available in pure form and be stable over a wide temperature range, and have reasonable prices. According to these factors, acceptable oxidizers include a variety of anions like nitrate, perchlorate and chlorate. The reaction of nitrate is XNO3 =XNO2 + 1/2O2, the reaction of perchlorate is XClO4 = XCl + 2O2, and the reaction of chlorate is 2XClO3 = 2XCl + 3O2. Some cations used to combine with these anions have to meet some requirements, The oxidizer must be quite low in hygroscopicity, or the tendency to acquire moisture from the atmosphere and The oxidizer’s positive ion (cation) must not adversely affect the desired flame color. (Conkling P51) The oxygen released by these oxidizers will immediately combine with reducing agents such as sulfur and carbon to produce hot and rapidly expanding gases- sulfur dioxide and carbon dioxide. The aim of reducing agents is to control the speed of reactions. A good fuel, or electron donor in firework, will react with oxygen that oxidizers produce to form a stable compound plus substantial heat in order to let “star” burn. Gunpowder is the commonly used fuel. It is composed of 75 percent of potassium nitrate, 15 percent of charcoal and 15 percent of sulfur. The reaction of burning gunpowder is 74KNO3+96C+30S+16H2O=35N2+56CO2+14CO+3CH4+2H2S+4H2+19K2CO3+7K2SO4+8K2S2O3+2K2S+2KSCN+ (NH4)2CO3+C+S.
Every time we watch fireworks with different colors because of the colorant agents. Colorant agents that are chemicals are used to produce different colored fireworks. These colors are produced by heating different metal salts and each of them will emit characteristic colors, which is in Figure 1. There are two main mechanisms of color production in firework. First one is Incandescence and second one is Luminescence. Incandescence is the emission of light by high temperature. The atoms of each element absorb energy and release it as light with particular colors. Luminescence is the emission by other factors instead of heating the substances. The energy an atom absorbed will rearrange its electron from ground state to excited state. The excess energy of excited state will emit as light as well. The amount of energy released varies from element to element, which is characterized by a specific wavelength of light. The higher energy an atom released, the shorter wavelength the light is. Conversely, the lower energy an atom released, the longer wavelength the light is, which is in Figure 2. Binder, which is the organic polymer, holds all components together in firework. The widely used binder is dextrin which also acts as a fuel. The other binders include nitrocellulose, polyvinyl alcohol and Laminac. A good binder should be neutral, non-hygroscopic in order to avoid water or vapor to affect the reactions of fireworks.
Figure 2, wavelength vs. Energy, anatomy of a firework contains seven parts which are in Figure 3. These parts make fireworks can explode successfully. The explosion of firework has two steps: Shell, which is made up of gunpowder, is shot into the air, and then will explode. In order to rocket shell into the air, the shell has to be placed in the tube called launch tube. A lifting charge of gunpowder sits below the tube with a main fuse attached. When the main fuse is ignited, the gunpowder in the lifting charge will be exploded and produce substantial gas plus heat, which will shoot the shell into the air. When the shell is higher enough, the time-delay fuse will be ignited and black powder(gunpowder) and stars (gives firework color) will explode. This process will produce lots of gases and heat in order to let shell open, propelling the stars into every direction. In addition, different styles of fireworks (Figure 4) have different structures. But they all have two important elements: black powder and stars.
Fireworks are used so frequently in celebrating holidays nowadays so that it is easy for us to forget that firework is one of the factors due to air pollution. By burning fireworks, there are several air pollutants caused by it-sulfur oxides, nitrogen oxides, carbon monoxide, particulate matter (PM2.5), and toxic metals. Sulfur oxides such as sulfur dioxide and sulfur ions, which are formed by reactions involve sulfur-containing compounds. In addition, sulfur dioxide and sulfur ions will be transferred to sulfuric acid(H2SO4) which will dissolve into the water vapor in atmosphere to lead to acid rain. Nitrogen oxides have the same effects as sulfur oxides. They will be transferred into nitric acid and dissolve into the water vapor in the atmosphere, which leads to the acid rain as well. Carbon dioxide is a colorless, odourless, but poisonous gas. When the carbon in firework is not burned completely, it will produce carbon monoxide. The reaction is 2C + O2 = 2CO. PM2.5, which is a tiny particle in the atmosphere, leads to horrible air pollution. Take the time during Spring Festival and New Year’s Day in Beijing, China as an example. Readings of fine particulate matter called PM2.5 reached 1.593 milligrams per cubic meter on the Jan. 22 eve of the holiday, about 100 times worse than the amount considered good for 24-hour exposure, the city’s environmental bureau said. (Bodeen,2012) In addition, some toxic metals like lead, copper and mercury will come from colorant agents to lead to air pollution.
Firework is one of the most amazing inventions and it plays an important role in celebrating holidays and some special occasions nowadays. The six components of firework contribute lots of efforts to the wonder of firework. The reactions between oxidizers, reducers and fuel make fireworks explode successfully. The colorant agents produce particular colors and light to make different colored fireworks. The binder holds everything together in order to make every reaction occur. In addition, in this paper, we get to know about the anatomy of firework and how does fireworks be rocketed into the air. Each part of firework makes it happen. Even though firework is incredible, it is one of the main factors due to air pollution. In this paper, we learn about this and should be aware of it.
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