When energy in the form of light is released from matter because of a chemical reaction the process is called chemiluminescence. One example of a common chemiluminescent reaction is a flame, where the reaction between a fuel and an oxidant produce excited state products that emit light; however, as an example of chemiluminescence this process is complicated by the fact that incandescent particles are often also present because of the amount of heat released by the reaction; therefore, some (or most?) of the light in common flame comes from very hot incandescent emissions.
A better example of a chemiluminescent reaction is between nitrogen monoxide (symbol NO) and ozone (O3). This reaction is routinely used to determine either ozone (using excess NO) or NO (using excess O3).
The reaction is shown in the following equations:
Nitrogen monoxide reacts with ozone to produce nitrogen dioxide (NO2) in an excited state (denoted by the raised asterisk). Little of the excess energy involved in this process is released as heat; therefore, the reaction mixture and products do NOT incandesce to any significant degree. The reaction produces an excited state NO2 which returns to a lower energy state by (in part) releasing photons of light: chemiluminescence. This electromagnetic radiation has a range of wavelengths; however, the emission is centered around 1200 nanometers (nm).
The conditional words in part are included in the last paragraph because there is actually two ways excited state NO2 can de-excite. One is via photon emission (chemiluminescence); another is by losing energy through collisions with other particles. This collisional process becomes more and more significant as the amount of particles available for collisions increases. In the gas phase, higher pressure means higher collision rate. This is why most gas phase chemiluminescence reactions are performed at low pressures; this increases the amount of energy released via photon emission by decreasing the amount of collisional deactivation.
More hypertext pages involving gas phase chemiluminescence are here.
The much larger world of liquid phase chemiluminescence is described on these hypertext pages.