Monday, July 04, 2016

The Chemistry of Fireworks (redux)

Three years ago (wow how time flies!) I posted about the Chemistry of Fireworks.  At that time, the post was just a quick note about how different elements produce different colored lights in the night sky.

I always wanted to expand upon that post ... and this year's 4th of July seemed like the perfect opportunity.  I found quite a few good infographics and some wonderful websites to help me along the way.  My sources are cited at the end.

"Fireworks generate three very noticeable forms of energy: a tremendous release of sound, bright light, and heat. The tremendous booms heard at ground level are the result of the rapid release of energy into the air, causing the air to expand faster than the speed of sound. This produces a shock wave, a sonic boom." 1

"Excluding propellants or special effects, the points of light ejected from fireworks, termed 'stars', generally require an oxygen-producer, fuel, binder (to keep everything where it needs to be), and color producer." 2 

The color of a firework is determined by the elements used in its "stars".  What happens is, when these elements lose energy during the explosion of the fuel, that energy is emitted as light.  Each element emits a slightly different wavelength of energy, and therefore a slightly different color of light results.

Taking you all the way back to high school chemistry, this is the Periodic Table of Elements and the aqua colored boxes are those elements that produce the spectacular colored firework we enjoy during celebrations.

"There are two main mechanisms of color production in fireworks, incandescence and luminescence.

Incandescence is light produced from heat. Heat causes a substance to become hot and glow, initially emitting infrared, then red, orange, yellow, and white light as it becomes increasingly hotter. When the temperature of a firework is controlled, the glow of components, such as charcoal, can be manipulated to be the desired color (temperature) at the proper time. Metals, such as aluminum, magnesium, and titanium, burn very brightly and are useful for increasing the temperature of the firework.


Luminescence is light produced using energy sources other than heat. Sometimes luminescence is called 'cold light', because it can occur at room temperature and cooler temperatures. To produce luminescence, energy is absorbed by an electron of an atom or molecule, causing it to become excited, but unstable. When the electron returns to a lower energy state the energy is released in the form of a photon (light). The energy of the photon determines its wavelength or color." 2

"The [incandescent] colors are produced by heating metal salts, such as calcium chloride or sodium nitrate, that emit characteristic colors. The atoms of each element absorb energy and release it as light of specific colors. The energy absorbed by an atom rearranges its electrons from their lowest-energy state, called the ground state, up to a higher-energy state, called an excited state. The excess energy of the excited state is emitted as light, as the electrons descend to lower-energy states, and ultimately, the ground state. The amount of energy emitted is characteristic of the element, and the amount of energy determines the color of the light emitted. For example, when sodium nitrate is heated, the electrons of the sodium atoms absorb heat energy and become excited. This high-energy excited state does not last for long, and the excited electrons of the sodium atom quickly release their energy, about 200 kJ/mol, which is the energy of yellow light.

"The amount of energy released, which varies from element to element, is characterized by a particular wavelength of light. Higher energies correspond to shorter wavelength light, whose characteristic colors are located in the violet/blue region of the visible spectrum. Lower energies correspond to longer wavelength light, at the orange/red end of the spectrum." 1

The Technical Details:

Click on the image to follow a link to the NOVA
 website for a break down of each part and how it
"In making fireworks, the metal salts are put into stars, small clay or dough-like lumps or cubes 3 to 4 cm in diameter. Stars consist of a blend of oxidizing agent, reducing agent, coloring agent (metal salt), and binders. When ignited, the stars produce both sound and light effects. The appearance of a firework is determined by its stars, which are made by hand and carefully packed into cardboard compartments within the firework shell, where they await ignition by a time-delay fuse. 

 "From lift-off to color release, a carefully choreographed sequence of events takes place, producing the desired effect. The power needed to lift each firework into the air is provided by the highly exothermic combustion of black powder, a slow-burning combination of 75% potassium nitrate, 15% charcoal, and 10% sulfur. Said to have first been used in China about 1000 years ago, the recipe for black (or coal) powder has undergone little change since then. This formulation explodes at a rate of about 3 meters per second, classifying it as a low explosive. In fact, when it burns in the open air, black powder’s heat and gas dissipate quickly. The key to fireworks’ success is to trap the heat and gas in the bottom of the shell, which is positioned in a launch tube or mortar, until the trapped gas pressure builds to such a force that, when it escapes, it hurls the firework high into the air.

 "A firework is ignited by lighting the main fuse. That simultaneously starts both the fast action fuse, which quickly carries the flame down to the lift charge, and the time delay fuse, which continues to burn upward toward the cardboard compartments containing the stars, even as the firework is hurtling skyward.

 "Fireworks are classified as both a low and a high explosive. The initial lift charge that sends the firework into the sky is a low explosive. The burning charge undergoes rapid decomposition, but not detonation. The firework can be thought of as flying through the air powered by a fast burning wick. Where the wick ends, it meets the high explosive components of the firework. In this second stage there is an instantaneous detonation producing both a loud explosion and a bright flash of color.

 "The black powder lift-charge is calculated to exhaust itself precisely when the slow-burning, time-delay fuse reaches the first compartment packed with light-producing stars and black powder. This happens when the firework is at the very apex of its upward flight. Simultaneously the fuse sets off sound-producing explosives and detonates the stars, initiating color emission. If the timing of the fuses is off, however, the firework may detonate early, too close to the ground, or late, when the firework is falling back to earth.

 "When an aerial firework explodes, its component stars fly off in all directions. However, when viewed from a distance, these aerial fireworks seem flat, as though they were displayed on a screen. We do not easily perceive that some parts are coming toward us, while others are moving away. We have a hard time seeing this, because we don't perceive the normal clues that tell us the direction in which something is moving. Normally, when an object moves toward us, it appears to grow larger, and when it moves away, it appears to grow smaller. However, the stars in fireworks are so bright against a dark background, that we can't get an accurate impression of what size they are; their intensity saturates our retinas. We can't tell if they are getting larger or smaller, so we judge them not to be moving either away from us or toward us. Therefore, they look flat. If, however, we could see them from directly below, we would observe that the stars move in all directions away from the central explosion.

"When watching fireworks, we see them much sooner than we hear them. That happens because light travels about a million times as fast as sound. The speed of light is 300,000,000 meters per second, but the speed of sound is only about 340 meters per second. If you are watching fireworks that are about a kilometer (1000 meters) away, the light takes only 3 millionths of a second to reach you. The sound takes about 3 seconds. You can tell how many kilometers away fireworks are exploding by starting to count seconds as soon as you see an explosion. Stop counting when you hear the explosion and divide the count by 3. This gives the distance away in kilometers." 1



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