Kimberly A. Lawler-Sagarin, Chemistry 100, Fall 2006

1. Color and the Chemistry of Fireworks Kimberly A. Lawler-Sagarin, Chemistry 100, Fall 2006 Introduction Luminescence Everybody enjoys fireworks on the fourth of July! Explosions, bright light and brilliant colors lead to beautiful displays that make millions “oooh” and “ah”. But did you know that chemistry is behind these amazing displays? To understand the role of chemistry, let’s take a look at two phenomena that are responsible for the colors of fireworks: incandescence and luminescence. • Luminescence is defined as the emission of light without heat. That is, any emission of light is not solely due to high temperatures. There are many forms of luminescence, some of which we will discuss later in the semester. The bright colors in firework displays are caused by the excitations of atoms or small molecules. • Atoms consist of two main parts: • The nucleus - the core of the atom. The nucleus consists of two types of smaller particles, protons and neutrons. Protons have a positive electrical charge, whereas neutrons are neutral, so the nucleus of the atom is positively chargee • Electrons - small negatively charged particles that exist outside the nucleus. • In our modern model of the atom, all the electrons in the atom do not have the same energy. Electrons occupy different energy levels. It is these energy levels that are behind the color of fireworks. • Essentially, an electron in a lower energy level can be promoted to a higher energy level by adding energy (such as during the explosion of a firework). Now, the atom has excess energy. However, this "excited" atom has a very limited lifetime. It quickly returns back to the lower energy state. One way it can do this is by emitting a photon. Incandescence Incandescence is the emission of light from a warm or hot object. We encounter many examples of incandescence in our daily lives. A common example is the (incandescent) light bulb. A tungsten filament is sealed into an evacuated glass bulb. When heated, the filament releases light. Other examples of incandescence are the "red hot" coals in a barbeque grill, the "red hot" or "white hot" pieces of glowing metal on might see the hands of a blacksmith in an old western movie, or even just the heated carbon particles in a candle flame. Below is a picture of one of our chemistry majors, Mike Zickus, burning magnesium metal. Magnesium metal (Mg) combines with oxygen (O2) to form magnesium oxide (MgO). The brilliant white light is produced from the high temperatures generated by the heat released in the reaction. Magnesium powder is used in many types of fireworks because of this property. This explains the intense light given off by fireworks, but to describe the colors, we must look to luminescence. Figures 2-3: Fireworks from the 2004 Batavia, Illinois fireworks show. Figures 4: The wavelength of light emitted has an energy that corresponds to the energy difference between the two levels. This determines the color we see. Atoms of different elements have different spacing between their energy levels, leading to different colors. Thus, a variety of different substances are used to produce the colors we see in a fireworks show. Figures 5: Here, methanol burns in the presence of various metal salts, each salt giving rise to a different color. Clockwise, from top: orange (sodium), yellow-green (barium), green-blue (copper), red (strontium), violet (potassium). Figures 6: Red fireworks are commonly created by lithium or strontium salts. Figure 1: Incandescence produced during the combustion of magnesium metal So, the next time you are enjoying a fireworks show, think of chemistry!. References: Nassau, K. The Physics and Chemistry of Color: The Fifteen Causes of Color; 2nd ed.; Wiley: New York 2001. Helmenstin, A. M. "The Chemistry of Firework Colors" http://chemistry.about.com/library/weekly/aa062701a.htm (accessed February 2005).