Apparatus

Bunsen burner, heat-proof mat.
Metal tray such as a large tea tray.
Watch glass



Chemicals

The quantities given are for one demonstration.
10g of ammonium dichromate (VI).
Wooden spill.
A little ethanol.
One piece of blue cobalt chloride paper.
Access to a fume cupboard.

The Demonstration

Working in a fume cupboard, place a conical pile of about 10g of ammonium dichromate on a heat-proof mat standing on a tray to collect the chromium oxide that shoots into the air. Soak about a 3cm length of wooden spill in ethanol and stick this into the top of the pile so that about 2cm protrudes to act as a wick. Light the wick.

As the wick burns down into the ammonium dichromate, the compound begins to spark and decompose leaving behind a cone of green chromium (III) oxide that has a considerably larger volume than the original compound. Some of this oxide shoots into the air.

The 'volcano' burns for between 30 seconds and one minute. Hold a watch glass above the 'volcano'; this becomes steamed up with water from the decomposition. Confirm that this is water with blue cobalt chloride paper.

The reaction may also be started by pointing a roaring Bunsen flame at the top of the pile of ammonium dichromate.