Ammonium dichromate, (NH4)2Cr2O7, decomposes when heated to produce chromium(III) oxide [Cr2O3], nitrogen gas, and water vapor:
(NH4)2Cr2O7(s) ——> Cr2O3(s) + N2(g) + 4H2O(g)
While the ammonium dichromate decomposes, it gives off orange sparks and throws the green chromium(III) oxide crystals into the air, producing an effect that looks like a miniature volcanic eruption.
The chromium(III) oxide crystals that are produced are "fluffier" than the original ammonium dichromate crystals, and even though a lot of the mass of the starting materials escapes as vapor, the product looks like a larger amount of material.
In the following demonstrations, a pile of ammonium dichromate is ignited with a Bunsen burner, producing the "volcano" effect. Notice the volume of the green chromium(III) oxide solid produced, and how far the crystals have been thrown. The second video clip shows a similar demonstration at a closer magnification.
Video Clip 1: REAL, 8.33 MB
Video Clip 2: REAL, 4.92 MB
!!! Hazards !!!
Chromium salts are irritating to the skin and respiratory tracts, and are also carcinogenic. (The ammonium dichromate contains chromium in the hexavalent, Cr6+ form, featured in the movie Erin Brockovich.) The best way to do this demonstration is on a large piece of aluminum foil, which can then be used to wrap up the chromium salts produced.
Decomposition of Ammonium Dichromate: Bassam Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1. Madison: The University of Wisconsin Press, 1983, p. 81-82.
Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.