In general, oxidation reactions release energy in form of heat as well as in form of visible light. In addition, also reactions generating light but almost no heat are also known. This phenomenon is called cold light or chemiluminescence. Cold light is not very intense and can, therefore, be better observed in the dark.
In the first experiment, chemiluminescence accompanying the oxidation of luminol (5-Amino-2,3-dihydro-1,4-phthalazinedione, C8H7O2N3) was demonstrated.
1) A solution of luminol in ammonia water (NH3·H2O) was poured in a flask and potassium tricyanocuprate(I) (K2Cu(CN)3) was added to it. Then, a solution of hydrogen peroxide (H2O2) was added to the mixture. Luminol was oxidised by hydrogen peroxide releasing energy as light flashes of light.
2) The glow sticks on sale probably contain hydrogen peroxide in external plastic ampoule and some fluorescent compound in internal glass ampoule. When the stick is bent, the ampoule breaks and the resulting reaction produces the glow seen in the dark.
Chemiluminescence can be observed in nature as well. In Nordic countries, for example, the green glow of the glowworm-Lampyris Noctiluca (beetle) is known, caused by the oxidation of a protein compound luciferin. It is proposed that ~98% of reaction energy is released in the 480–720 nm wavelength light region and only 2% as heat.