The ammonium dichromate volcano is a classic chemistry demonstration. This particular chemical volcano also goes by the name “Vesuvius fire.” It’s the most realistic chemical volcano, with glowing orange embers, steam, sparks, and lots of dark green ash. The volcano raises interest in chemistry and is an excellent example of a decomposition reaction, an exothermic reaction, and a chemical change. Here’s how to perform the ammonium dichromate volcano demonstration safety and a look at its chemical reactions.
Chemistry of the Chemical Volcano
Orange ammonium dichromate decomposes into green chromium(III) oxide, water vapor, and nitrogen gas.
(NH4)2Cr2O7 → Cr2O3 + 4 H2O + N2
Initially, the ammonium dichromate crystals darken, probably due to loss of ammonia and anion condensation associated with the oxidation state change to form chromium(III) oxide. The chemical volcano probably has molten “lava” in the form of liquid CrO3, but this intermediate (if it occurs) is transient.
The product of the reaction, Cr2O3, goes by the names “viridian” and “chromia.” It is used as a pigment in ink, paint, and glass.
Ammonium Dichromate Volcano Materials
Traditional
- ~20 grams of ammonium dichromate
- Sand tray or ceramic tile
- Fume hood
- Alcohol or acetone
- Lighter
Setting up and performing the demonstration is extremely simple.
- Pour the ammonium dichromate in a volcano-shaped pile on the sand or tile.
- Dampen the powder with a fuel, such as alcohol or acetone.
- Ignite the fuel using a lighter. Once ignited (180°C), the reaction is self-sustaining (~225°C).
This works fine, but the green dust gets everywhere, making perfect clean-up difficult. Fortunately, there is a simple solution to this concern, making the ammonium dichromate volcano safe for demonstrators, viewers, and whoever uses the facility next!
Safer Method
The ammonium dichromate volcano demonstration appears in high school and college chemistry labs, but isn’t found in modern chemical volcano kits because it involves a hexavalent chromium compound. The 6+ oxidation state of chromium is toxic, while the 3+ oxidation state is essential for human nutrition (but still toxic in high doses). The “eruption” changes Cr6+ into Cr3+ to form the oxide, but a small amount of ammonium dichromate remains. So, it’s important to collect the remains of the demonstration. Fortunately, it’s easy to perform a more environmentally-friendly eruption!
- ~5 grams of ammonium dichromate
- Large flask
- Glass tube or pipette
- Porcelain funnel or mineral wool
- Bunsen burner or gas flame
- You can pour the ammonium dichromate into the flask, but you’ll get a better “volcano” shape if you pour through glass tubing to control chemical placement. This is worth doing, since the cone shape burns more slowly and puts on a better display.
- Either plug the top of the flask with mineral wool or else cover it with an inverted glass or porcelain funnel. Remember, the reaction produces heat, so choose a nonflammable covering.
This version of the demonstration uses less material, so it generates less waste. After the contents of the flask have cooled to room temperature, simply pour the ash into a suitable storage container.
Safety Information
Wear gloves and safety goggles when working with ammonium dichromate (or any chromium compound, really). Work in a well-ventilated area, preferably under a ventilation hood.
References
- Basu, A.; Saha, B. (2010). “Kinetic Studies on Hexavalent Chromium Reduction.” Am. J. Anal. Chem. 01(01). doi:10.4236/ajac.2010.11003
- Freeman, F. (2001). “Chromic Acid”. Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons. doi:10.1002/047084289X.rc164
- Neugebauer, C. A.; Margrave, J. L. (1957). “The Heat Formation of Ammonium Dichromate”. J. Phys. Chem. 61 (10): 1429–1430. doi:10.1021/j150556a040
- Shakhashiri, B.Z. (1986) Chemical Demonstrations: A Handbook for Teachers of Chemistry. Vol. 1. University of Wisconsin Press. pp. 81-82.
- Young, A.J. (2005). “CLIP, Chemical Laboratory Information Profile: Ammonium Dichromate”. J. Chem. Educ. 82 (11): 1617. doi:10.1021/ed082p1617