Further Exploration Activities

Fireworks: What Do We Know About Fireworks?

Student Questions (Worksheet 3) (20-25 minutes)
Questions appropriate for class work or for homework. Answers are provided.

Use Think-Pair-Share to involve all students in the discussions. First ask students to think about the questions silently, then ask them to share their ideas with a neighbor. Finally, discuss their answers as a class.

Talking Points For Follow Up Class Discussion (10-15 minutes)

Note: When the teacher returns, if a quick discussion follow up to the classwork is desired, the teacher could use related questions to reinforce what the students learned by reading the article:

1. What is inside a firework?

2. How do they now know the different colors of fireworks are produced?

3. How are the different firework shapes produced?

4. Where does the sound of fireworks come from?

5. What safety issues should be considered when creating or using fireworks?

6. What did they find most interesting or surprising about their reading?

Talking Points for Relating the Article to Chemistry Concepts (10-15 minutes)

1. Give examples from the article of physical and chemical changes.

2. What aspects of fireworks relate to gas laws?

3. What kind of chemical reaction produces the explosion?

4. How are the colors of fireworks related to electron structure and atomic theory?

5. What insights did they gain about safety?

Have students write a one-sentence summary (20 words maximum) or a “Tweet” (140 characters) of the most important chemical idea they learned from reading the article.

Further Explorations Online (Worksheet 4) ((30 -45 minutes)
Students will work in groups to research fireworks and find three amazing facts or stories that they would have included in the article about fireworks.

Background Information for the Teacher

History of fireworks

The discovery of fireworks parallels the discovery of gunpowder. All of the important chemical substances needed to make gunpowder—saltpeter (KNO3), sulfur and carbon—were found in ancient China. The Chinese used saltpeter as a food flavoring, in curing meat and in medicines for centuries prior to the discovery of gunpowder. The compound was found in southern China in much greater abundance than in Europe, and it is no surprise, then, that gunpowder was discovered in China.

Early alchemists in China experimented with combinations of substances to find an “elixir of immortality.” Among the early combinations was a mixture of saltpeter and sulfur, both of which were used medicinally in China. As early as the second century, alchemists in China knew how to purify sulfur by heating iron pyrite (FeS2). Historical records indicate that Tang Dynasty alchemists in the 9th century were mixing saltpeter and sulfur with carbon in their search for the elusive elixir. The records also show that the mixture was found to be flammable and often explosive.

In 1040 CE, Chinese alchemist Tseng Kun-Liang published the first gunpowder formula for use in weapons. The mixture was not, in fact, explosive because it did not contain a sufficient amount of potassium nitrate. Eventually it was discovered that the gunpowder mixture needed to be 75 % potassium nitrate in order to explode. The typical modern formulation for gunpowder is 75% saltpeter, 15% charcoal and 10% sulfur. By the 13th century, during the Song Dynasty, the Chinese were using gunpowder in military weapons.

Early fireworks existed as long ago as 250 BCE when records indicate that the Chinese filled bamboo shoots with some type of explosive substance to produce what we would call firecrackers. There is the story of the Chinese cook who inadvertently dropped saltpeter into the fire. Early fireworks likely started in the form of firecrackers produced by the monk Li Tian in Hunan Province using bamboo shoots containing gunpowder. The firecrackers had religious significance in that they produced a loud noise on explosion. The noise was thought to ward off evil spirits. By the 15th Century fireworks had become part of religious ceremonies, military celebrations, and weddings.

Military processions were often led by men carrying clubs or lances with fireworks attached to the end. They were called “Green Men” because they covered themselves with green leaves from trees in order to shield themselves against the sparks from the fireworks. Gunpowder inevitably found military applications. By the 13th century the Chinese were using gunpowder to propel arrows at Mongol invaders. This led to aerial fireworks in addition to firecrackers. In modern fireworks, gunpowder is used to launch the aerial displays in addition to causing the shells to explode.

Fireworks spread to Europe and the Middle East and in the 14th century there were important developments in fireworks in both Italy and Germany. Italian pyrotechnicians developed aerial displays with stunning colors and other effects, while their German counterparts were advancing the science behind fireworks. The Italian interest is due in part to the fact that Marco Polo brought back firecrackers from the Orient in 1292. The Italians developed aerial shells, using gunpowder to both launch and explode the aerial shells. They added metal particles to create gold and silver sparks. In the 1700s it was also the Italian pyrotechnicians who began adding metal salts to fireworks. It is interesting to note that three of the leading companies producing fireworks are owned by families of Italian descent—the Grucci’s, the Rozzi’s and the Zambelli’s.

By the mid-1600s fireworks were immensely popular throughout Europe and especially Great Britain, some of it due, no doubt, to a 1634 publication by John Bate titled “The Mysteryes of Nature and Art: Conteined in foure severall Tretises,” The second treatise was titled “Fyer works” and it described in detail how to make various fireworks of the day. Below is pictured the cover of the second treatise with a “green man” illustration and a page full of diagrams from the book.

The record of Anne Boleyn’s coronation procession in 1533, says "At the head of her procession was an enormous fire breathing dragon followed by a green man casting fire and making a hideous noise". Queen Elizabeth I enjoyed fireworks so much she created a government position called “Firemaster of England.” Shakespeare wrote about fireworks. By the mid-1800s their popularity had spread to the United States.

Until the 19th century fireworks were essentially noisemakers and did not have the spectacular visuals effects we know today. About this time compounds that imparted color to the displays were added to the shells.

Fireworks are very popular in the United States today. According to the American Pyrotechnics Association, a trade group, there were 213.2 million pounds of fireworks sold in the U.S.—186.4 million pounds of that for consumer use and the rest for use in public displays. Industry sales increased from $425 million in 1998, to $940 million in 2008. The Walt Disney Company is the largest consumer of fireworks in the U.S.

Chemistry of fireworks

There are three types of fireworks—aerial displays, sparklers and firecrackers. Regardless of whether the pyrotechnics are used in the air or on the ground, they require the same four basic kinds of chemical substances—an oxidizer, a fuel, a colorant and a binder. Gunpowder—the basic fireworks staple—contains the fuel and the oxidizer. Metal salts are used as the colorants.

There are structural differences between the three types. Firecrackers are simply gunpowder wrapped in paper with a fuse attached. Aerial fireworks contain the four types of chemical listed above. They are sent into the sky using a lift charge of gunpowder, which also lights a time-delay fuse. When the shell reaches the right height the fuse ignites the gunpowder break charge, scattering the stars, which are themselves made of the four basic chemicals. The burning stars create the light show we associate with fireworks. Sparklers are made from a dried slurry of gunpowder and metal powder with a wire to support the mixture. Each of the three types of fireworks relies on rapid combustion.

As your students will know, combustion requires a supply of oxygen, and if that oxygen comes from the atmosphere, its concentration will be relatively low since the atmosphere is only about 20% oxygen. In order to produce a reaction rate rapid enough to be an explosion, fireworks require their own oxygen supply in the form of a chemical oxidizer. So a fuel and an oxidizer are two of the basic components required for fireworks. Where does the color come from? Metals or metal salts are the colorants in most fireworks. So let’s look a little more closely at the chemistry of these three components.

Oxidizers These are the oxygen-rich compounds needed to produce fireworks explosions. Chemical compounds typically used as oxidizers in fireworks are nitrates, chlorates and perchlorates. Potassium is often the anion of choice because the pale violet color it produces as it burns does not mask or interfere with other colorants.

Nitrates, which are the main component of gunpowder, decompose in the explosion to produce potassium oxide, nitrogen and oxygen according to the equation:

2 KNO3 à K2O + N2 + 2.5 O2

Note that the nitrate ion gives up only two of its three oxygen atoms. Therefore, the reaction is more controlled and occurs at a lower temperature. Because of this, gunpowder is used primarily in the lift charged use to send the shells into the air and in the ignition of star bundles, but not in the ignition of the stars themselves. Other oxidizers are used in the star bursts to provide a higher temperature explosion.

By the early 1800s Italian pyrotechnicians discovered that chlorate oxidizers could produce temperatures near 2000oC, thus producing more intense star colors. Chlorates give up each of their three oxygen atoms, making them better oxygen sources than nitrates:

2 KClO3(s) à 2 KCl(s) + 3 O2(g)

The problem with chlorates is that they are less stable than nitrates, making them more dangerous to handle. Because of this instability, many fireworks manufacturers are using perchlorates as oxidizers. Perchlorates are more stable than chlorates but have the advantage of releasing all four oxygen atom in the reaction:

KClO4(s) à KCl(s) + 2 O2(g)

Fuel Carbon and sulfur are common fireworks fuels. Chemically they act as reducing agents, combining quickly with the oxygen released by oxidizing agents:

C(s) + O2(g) à CO2(g)

S(s) + O2(g) à SO2(g)

Note that the products of the reactions above are gases, which are at very high temperatures and, therefore, expanding rapidly. This is what creates the explosion. In addition to mixing the correct chemicals, pyrotechnicians also vary the particle sizes and the proportions of each. For example, in order to slow down the burning slightly to create a certain effect, they will grind the compounds to a particle size in the range of 250–300 µm. In this case they might also mix the compounds loosely so that the fuel and the oxidizer come in contact slowly. Or, for those aerial blast sparklers, technicians might increase the particle size in the mixture to 1000 µm so that the stars (the pockets of colorants within the shell) are barely ignited by the gunpowder and then combine more slowly with oxygen in the air to create a lasting effect. In addition, reaction rates in fireworks are affected by the presence of accelerators like sulfur or sugar or retarders like salt, packing density and moisture content.

Colorants In general there are two ways in which color is produced in fireworks—incandescence and luminescence.

The stars, those pellets of metal salts embedded in the aerial shell, produce color by luminescence. In addition to metal salts, the stars contain gunpowder (the fuel), an oxidizing agent and a binder. The stars are embedded in the gunpowder of the main shell. When the shell is sent skyward and the bursting charge explodes, the stars are ignited and are sent scattering. Some of the heat from the bursting charge—temperatures can reach in excess of 2000oC—is absorbed by the metal salts in the stars. Electrons in the metal ions contained in the salts absorb heat and move from their normal ground state to a higher energy level—called an excited state. Immediately these electrons return to ground state and release the energy they originally absorbed in the form of a photon. In this way light is produced by luminescence.

The energy released corresponds to specific frequencies of light in the electromagnetic spectrum, which can be calculated using the equation:

E = hn E = energy

h = Planck’s constant (6.626 x 10-34 m2kg/sec)

n = frequency

The equation allows you to calculate the frequency of light emitted by the photon. The emitted frequencies are, in fact, a signature for the ions involved. Each frequency of light is visible when viewed through a spectroscope, but when viewed by the naked eye the light appears to be a single color or blend of related colors. The visible colors are not, in fact, colors of a single frequency but are a blend of colors, made up of multiple frequencies. Each metal has its own unique “fingerprint” of frequencies, resulting in a color associated with the element. These are listed in the table below. Compounds of these materials are the colorants in fireworks. Some colors associated with metals used in fireworks stars:

Color Element Compound(s)

Blue-Green Copper copper acetoarsenite, copper (I) chloride

Violet Potassium potassium chloride

Red Lithium lithium carbonate

Crimson Strontium strontium carbonate

Orange Calcium calcium chloride, calcium sulfate

Yellow-Green Barium barium chloride

Yellow Sodium sodium nitrate, cryolite

Below is a table of colors and associated wave lengths:

Color Wavelength (nm) Frequency (THz)
Red
780–622 384–482
Orange
622–597 482–503
Yellow
597–577 503–520
Green
577–492 520–610
Blue
492–455 610–659
Violet
455–390 659–769

A video of typical flame colors can be viewed at http://www.youtube.com/watch?v=jJvS4uc4TbU.

In addition to producing color by luminescence, fireworks also produce color by incandescence. When a substance is heated, it gives off electromagnetic radiation, first in the infrared region, then, then red, orange, yellow and then white light. Flakes of metal like aluminum, magnesium and titanium are sometimes embedded in fireworks, especially sparklers and firecrackers. When they burn they glow and produce sparks and varying colors of light.

Firecrackers and Sparklers

Firecrackers were likely the earliest form of fireworks. They are simply gunpowder wrapped in paper. When the combustion reaction takes place in a firecracker the gaseous products expand and blow apart the paper wrapper. Sparklers are a little more complex. They are made up of a fuel, an oxidizer, metal powder and a binder. The components are mixed with water to form a slurry. The slurry is formed on to a wire, and when the slurry dries, a sparkler is the result. When sparklers are ignited at one end they burn slowly, giving off sparks, which are the result of the glowing metal powder—aluminum, magnesium or titanium. The light is given off by means of incandescence, described above.

Fireworks Safety

Fireworks are classified as explosives, and are, therefore, dangerous. This article provides an opportunity for you to reinforce all lab safety with your students.

Explosives can be classified by their sensitivity. Sensitivity refers to the amount of energy needed to initiate the reaction. The energy required to initiate the explosion can be delivered in a variety of ways—shock, impact, friction, electrical charge or detonation. All explosive reactions are exothermic, but most exothermic reactions are not explosive. The energy released in an explosion is primarily released in the form of heat, but just because a reaction releases a lot of heat does not mean it is necessarily explosive. One key requirement is that the release of heat must occur very rapidly. This rapid release of heat is critical. Explosive reactions produce gaseous products. When an explosive reaction takes place, the heat is released so rapidly that the temperature of the gaseous products rises suddenly. Thus, in accordance with Charles’ Law, the gases expand rapidly, one of the key features of an explosion. The result is called a detonation wave. The wave compresses and heats the gases immediately behind the front of the shock wave and the explosive reaction of these gases perpetuates the wave.

In the United States it is the Bureau of Alcohol, Tobacco, Firearms and Explosives that regulates the shipping of fireworks. It should be noted that the BATF regulations refer only to the shipping of fireworks and not their use. The U.S. Consumer Product Safety Commission regulates the sale and use. Most states also have their own laws regulating the sale and use of fireworks.

Class 1.1G (Mass Explosion Possible: Pyrotechnics)—general fireworks

Class 1.2G (Projection but not mass explosion: Pyrotechnics)—fireworks

Class 1.3G (Fire, Minor Blast: Pyrotechnics)—most display fireworks

Class 1.4G (Minor Explosion Hazard Confined To Package: Pyrotechnics)—Consumer or Common Fireworks (Most popular consumer fireworks sold in the US.)

The state laws governing fireworks can be accessed here: http://www.americanpyro.com/State%20Laws%20%28main%29/statelaws.html.

Connections to Chemistry Concepts (for correlation to course curriculum)

1. Redox—The chemical reactions that take place in a typical aerial fireworks display are oxidation-reduction reactions. The redox aspect of fireworks chemistry is not mentioned in the article but you can make this connection with your class.

2. Reaction types—Many of the reactions involved in fireworks are combustion reactions.

3. Thermochemistry—The relationship between the chemical reactions involved in fireworks and the energy produced by them is a major emphasis in the article.

4. Spectroscopy—Colorants in fireworks displays produce colors by luminescence—the result of excited electrons emitting specific frequencies of electromagnetic energy as they fall back to ground state. You can relate this to flame tests and include spectroscopes if you choose.

5. Gas Laws—Charles’ law is at work in the explosive reactions.

6. Safety—Safety is a major concern with fireworks. You can use this to stress general safety when using chemicals.

Possible Student Misconceptions (to aid teacher in addressing misconceptions)

1. “I thought combustion, like explosions, just required oxygen from the air. Why do fireworks need an oxidizer?” Combustion does rely on oxygen from the air. However, the atmosphere is only about 20 per cent oxygen, and fireworks need a higher concentration of oxygen in order to explode on schedule. So oxidizers are part of each aerial fireworks shell in order to supply the extra oxygen needed.

2. “Do they put dyes in the fireworks to produce the colors?” No. The chemicals in fireworks that produce the colors you see are called metal salts. When the fireworks explode the resulting heat excites electrons in the metal ions. The electrons quickly release the energy they absorbed in the form of colored light. For a more detailed explanation see “More on colorants.”

Anticipating Student Questions (answers to questions students might ask in class)

1. “The article explains how the colors are formed, but what causes the sound when fireworks explode?” As the article states, the explosion that produces the colors we see in fireworks is actually a chemical reaction in which the products are primarily gases. The temperature of the gases produced is around 2000oC. At those temperatures the gases expand rapidly (according to Charles’ Law) causing them to expand faster than the speed of sound. The noise we hear is actually a sonic boom.

2. “Why do I sometimes see the fireworks before I hear the sound of the explosion?” The reason that fireworks viewed from a distance are seen before the accompanying sound is heard is the difference in the speed of light and the speed of sound. Light travels at 300,000,000 meters per second and sound travels at about 340 meters per second. So for fireworks viewed at a distance of 1000 meters, the light would take about three millionths of a second while the sound would take nearly three seconds to reach you.