Ammonium Dichromate Volcano

A pile of ammonium dichromate is burned and decomposes exothermically to produce green chromium(III)oxide in dramatic fashion.

Equipment

  • large glass container (optional, but this reaction spews chromium oxide everywhere so it is recommended)
  • filter paper (for a wick) and some masking tape
  • funnel

Reagents

  • ~12 g. ammonium dichromate
  • ethanol

Presentation

  1. The wick is folded up so that it is long and thin and pushed through the nozzle of the funnel.
  2. The wick is taped to the bottom of the large container, still threaded through the nozzle of the funnel.
  3. While holding the funnel upright and pressed to the bottom of the container surface, pour the 12 g of ammonium dichromate into the funnel.  Slowly lift the funnel, tapping as you do so to keep the ammonium dichromate falling past the wick.  As you lift it up, the dichromate should form a conical mound with the filter paper wick sticking up out of the middle (still taped to the bottom).
  4. Douse the paper wick with ethanol.  Light it.

Details

The decomposition reaction is:

(NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4H2O(l)

As the wick burns down into the ammonium dichromate, the orange solid begins to give off sparks and decompose into chromium(III) oxide.  Chromium(III) oxide is a flaky green solid that has a considerably larger volume than the original compound. Some of this oxide shoots into the air during the reaction giving the appearance of an erupting volcano.  In fact, sometimes people put make a little pile of black sand and pour the dichromate into a crater in the middle so that all of the "eruption" appears to be coming from the top of a black mountain.

This is a redox reaction in which chromium is reduced from its +6 oxidation state in dichromate to its +3 oxidation state in the chromium trioxide.  Despite the sparks that emerge during the reaction, it is not a combustion reaction -- you could carry out the reaction in the absence of oxygen (by flushing the container with nitrogen) and it would proceed in the same way.

Safety

fire

hot

projectile

toxic

Special Safety Notes

  • Wear eye protection and avoid skin contact with ammonium dichromate (wear gloves).  Ammonium dichromate, is very toxic and is also explosive under certain conditions. 
  • The green chromium(III) oxide is low hazard but there may be traces of unreacted ammonium dichromate dust in the residue after the reaction. Wear gloves when transferring or sweeping up the residue into a plastic bag. 
  • Ethanol is highly flammable, use care.
volcano1
A photo of the volcano, not in a container, getting ready to make a mess on someone's bench top.
vol3
Attempting to capture the volcano in action.
The chromium(III)oxide aftermath.
The chromium(III)oxide aftermath.