Q: Where does the green “volcano” come from?

This is an easy enough demo to perform- you create a small pile of ammonium dichromate in a mortar or similar container, light the top of the pile, and watch the science happen.  The effect looks like a miniature volcano.  So what’s happening here?

This “volcano demo” starts with a simple pile of ammonium dichromate ((NH4)2Cr2O7), a bright orange power.  By applying heat and igniting the powder, the ammonium dichromate decomposes exothermically, breaking down into several products and giving off sparks and more heat.  The reaction produces nitrogen gas and water vapor, as well as the dark green “ash,” chromium (III) oxide.

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

Ammonium dichromate is mainly known for pyrotechnics (such as in this demonstration), but it has been used in photography in the gum bichromate process to print images from negatives.  Chromium (III) oxide is used as a green pigment.

You may notice that chromium (III) oxide has a mass of 151.99 grams per mole, which is lower than the mass of ammonium dichromate at 252.07 grams per mole.  (A mole is a unit of measurement of a substance.  One mole contains 6.022 x 1023 atoms or molecules.  For example, 6.022 x 1023 molecules of ammonium dichromate would have a mass of 252.07 grams.)  Yet the volcano seems to grow, even with the loss of mass as nitrogen and water vapor escape to the air.  This is because the chromium (III) oxide formed is flaky, taking up a larger volume than the original ammonium dichromate and increasing the “volcano” size.

Something I should point out- as awesome as this demo is, it should really only be done by professionals trained in this demonstration and within a controlled and well-ventilated environment.  Ammonium dichromate is a known human carcinogen, meaning it could possibly lead to cancer.  Lots of things are considered carcinogens- outdoor air pollution, processed meat, mineral oil, alcoholic drinks, and wood dust, just to name a few.  Still, it’s best to limit yourself from exposure to ammonium dichromate.  It can corrode mucous membranes of the nose, throat, or respiratory tract if inhaled, and irritate skin if touched.  (Chromium (III) oxide is generally not considered a health hazard, but you still shouldn’t eat it or anything.)

Oh, and the demonstration can be a bloody mess.  I remember my college chemistry group performing it at a demonstration show, and we misjudged how much ammonium dichromate we needed to start with.  It ended up getting larger than expected, and quite a bit of chromium (III) oxide ended up on the stage floor.  And even though the chromium (III) oxide itself wasn’t dangerous, the clean-up crew wasn’t too thrilled with us.  So we weren’t allowed to do it the next year.  Oops.

Keep calm and science on.

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