For Fourth of July excitement, it’s hard to top the celebration on the Washington D.C. Mall. In addition to the spectacular fireworks display, young children by the thousands carry light sticks or wear glowing bracelets and necklaces. The glowing lights are mostly green, but other colors are seen as well. Light given off by these devices is known as “cold light,” or chemiluminescence, and is the result of a chemical reaction.

Light sticks consist of a small glass tube inside an outer sealed, plastic tube. The small glass tube contains hydrogen peroxide (H₂O₂). The outer tube contains a phenyl oxalate ester and a dye. By bending the outer plastic tube, the smaller glass tube is broken, releasing the hydrogen peroxide. The peroxide reacts with phenyl oxalate, producing phenol and carbon dioxide. Energy from the reaction is transferred to the dye, which becomes activated and gives off light, as shown in the equation (1).

In chemiluminescence, the energy to excite electrons comes from the chemical rearrangement of atoms to form new molecules with new bonds. Just as electrons in atoms become excited to higher energy levels by supplying energy, electrons within molecules become excited to higher energy levels by supplying energy. Just as atoms give off light when the electrons drop to a lower energy level, the same happens when electrons drop to a lower energy level within molecules. The energy levels are of different energy within the molecules than in atoms. Since no heat is generated, energy given up as light by this means is sometimes called “cold light.”

Glow-in-the-dark light sticks aren’t the only use for chemiluminescent molecules. For instance, they are used to detect atmospheric concentrations of nitrous oxide (NO), a component of car exhaust. An air sample is pumped into a chamber, where the nitrous oxide reacts with ozone (O₃). Nitrogen dioxide (NO₂) is formed in an excited state (see equation), then it releases photons, or light energy. An instrument called a photomultiplier tube detects the light and amplified to give a measurable readout. Since 1 mole of NO yields 1 mole of NO₂, the light emitted is proportional to the concentration of the nitrogen dioxide formed and the initial amount of nitrous oxide (3).

* Excited state of NO₂

One experiment you might try is to place one end of a light stick in a cold water bath and the other end in a warm water bath. How do you think the light intensity on each side will compare? (See answer at the bottom of the page.) To learn more about this topic, see the Chemiluminescence Home Page for a number of QuickTime movies and a bibliography for further study (4). You can also create a cold light reaction with luminol, another chemical that reacts with hydrogen peroxide. (5)

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