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A Chemical Volcano:
The Decomposition of Ammonium Dichromate

 

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.

 

 

Procedures

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.

 

 

References

Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.