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The tabletop volcano, known under many other names like volcano experiment, volcano demonstration, dichromate volcano, AD volcano, thermal tabletop volcano, ammonium dichromate chemical volcano experiment, Vesuvian Fire, etc. is a simple chemical experiment which involves igniting a small pile of dry ammonium dichromate and its thermal decomposition causes it to spew flames and Cr(III) oxide particles in a manner similar to the eruption of a volcano.
How to do the classical experiment
First, make a conical or hemispherically mound or ammonium dichromate. Ignite the top of the pile using a fire source and let it burn. As it burns, the ammonium dichromate decomposes releasing nitrogen gas, water vapors and voluminous chromium(III) oxide "ash", the latter reminiscent of the pyroclastic flows from a volcano.
- (NH4)2Cr2O7 → Cr2O3 + N2 + 4 H2O
While rarely encountered, ammonium chromate should also be suitable for this experiment.
Thermite can be used instead of ammonium dichromate, however this version is even more dangerous than the classical one. You will also need a slightly different volcano design, to account for the extreme high temperature and a way to allow the molten metal to flow out of its "crater".
Mixing baking soda with vinegar (or some other organic acid) and a food coloring atop a conical mound will can be used as a "family-friendly" volcano, albeit this one is less spectacular. The same effect can be achieved by adding a porous material, like Mentos in carbonated drinks.
An interesting type of tabletop volcano can be made by igniting crushed sulfur. The sulfur melts and turns red, similar to lava, and ignites, giving a blue flame. This volcano will only burn as long as the sulfur is melted and the heat generated from the sulfur burning is insufficient to keept the sulfur molten, meaning it will not work for long and wither needs to be constantly reignited or the temperature of the sulfur should be kept at around 100 °C, to maintain a constant high temperature. Biggest drawback of this type of tabletop volcano is that it gives off highly toxic fumes of sulfur dioxide.
Safety and handling
Ammonium dichromate is carcinogenic, so make sure you do this reaction outside or in a fumehood. Not all ammonium dichromate decomposes during the process, so to neutralize any leftover Cr6+, wash the residue with a reducing agent, like sodium/potassium sulfite, sodium/potassium metabisulfite, sodium thiosulfate or ascorbic acid.
Wear gloves when doing this experiment.