Ammonium Dichromate Volcano
Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff
Categories: Chemical Reactions, Oxidation and Reduction
Alternative titles:
Summary
A small mound of ammonium dichromate is ignited so it decomposes with sparks and a dramatic color change from bright orange crystals to dark green chromium(III) oxide, resembling a miniature volcano.
Procedure
Place a heatproof tray or foil-lined ceramic tile inside a working fume hood and set a small, conical mound of ammonium dichromate at the center.
Form a shallow crater at the top of the mound and insert a small ignition source such as a few match heads or a preheated metal wire.
Ignite remotely using a long lighter or by energizing the hot wire, then step back and observe the sparks, gas release, and the pile expanding into a fluffy green solid.
Allow the setup to cool completely before handling any residue.
Links
Variations
Use a smaller micro-scale volcano on a watch glass to reduce material and exposure.
Replace match heads with an electrically heated nichrome wire for controlled, hands-off ignition.
Safety Precautions
Conduct all chromium demonstrations in a certified fume hood. Avoid inhalation of dust and vapors.
Wear splash goggles, lab coat, and appropriate chemical-resistant gloves; tie back hair and remove ignition sources from the area.
Ammonium dichromate contains hexavalent chromium, which is toxic and a known carcinogen. Minimize quantities and avoid skin contact; do not allow any residue to dry and become airborne.
Chromium(III) oxide residue and any contaminated foil, paper, or tools must be collected as hazardous waste; do not sweep dry or dispose to drains.
Keep combustible materials away from the volcano setup; hot solids can ignite nearby items.
Use remote ignition methods when possible and keep observers at a safe distance behind a shield.
Decontaminate surfaces with damp disposable wipes that are then placed in hazardous waste; wash hands thoroughly after the activity.
Questions to Consider
What are the oxidation states of each element before and after the reaction, and which species are oxidized or reduced?
What gaseous products drive the “eruption” and fluffiness of the residue in the volcano? (Water vapor and nitrogen gas; the gas flow lofts and disperses the forming chromium(III) oxide.)
Write the balanced decomposition equation for the volcano and identify the physical states. ((NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4 H2O(g); solids form the residue, gases provide the eruptive effect.)