- Take a piece of metal cerium.
- Quickly move this piece on the surface of sandpaper.
- Observe sparks.
Sparks appear from friction of metal on sandpaper
Scientific name: Pyrophoric properties of cerium
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6 Ce + 5 O2 → 2 Ce2O3*CeO2
Cerium powder is formed by friction of the sandpaper. The particulate cerium is pyrophoric, it ignites spontaneously in air. The contact surface of the metal with oxygen increases during grinding. Cerium atoms located at the surface of the particle combine with oxygen in the air and are oxidized. The reaction is exothermic, some of the energy is transmitted to the neighboring particles of cerium, they pass into the excited state, and the reaction rate with oxygen increases.
This does not occur with the unground piece of cerium because of the distribution of heat energy in a large volume of material and the inability of cerium atoms to react with oxygen, which are not in contact with air.