Green Volcano

Chemistry:	Thermal Decomposition, Oxidizers
Grade Range:	Middle School, High School
Format:	Stage

This demonstration is exciting to watch, and makes a good addition to lessons on combustion reaction types. The salt used in this demonstration is toxic, so follow all directions provided in the Safety Precautions section of this write-up.

Materials

• Ammonium Dichromate Salt
• Large Watch Glass or Petri Dish
• Matches
• Plastic Tablecloth
• Scoopula
• Optional: White Board with Markers

Safety Precautions

Please read the Dry Chemical and Fire Demonstration sections of the Demonstration Safety page before performing this demonstration. Ammonium Dichromate is highly toxic and a known carcinogen, so avoid direct contact with the salt. The resulting product, Ammonium (III) Oxide, is nontoxic and safe to dispose of in a trash bag. The product should not be directly handled, however, to avoid contact with any non-reacted salt.

This demonstration requires: safety goggles, disposable gloves, small blast shield

Demonstration

1. Lay out the plastic tablecloth on the table, and put the watch glass or petri dish in the center. Put on the gloves and safety goggles, and set the small blast shield in front of the dish. Use the scoopula to scoop out some of the salt onto the dish, making a small mound. The mound should be centered in the dish, with a minimum of ¹/₂ in of room between the edges of the mound and the edge of the dish.
2. Explain to the audience that the orange salt on the dish is Ammonium Dichromate, a strong oxidizing agent. State that unlike other oxidizers like H₂O₂, Ammonium Dichromate will quickly decompose without a catalyst.
3. Show the matches and ask for a countdown. At the end of the countdown, light a match and hold it above the top of the mound. After warming for a few seconds, it will start sending out sparks!
4. Back away and let the reaction run. The mound will send out orange-red sparks, and will start to make a pile of green flakes! Depending on the size of the mound, this reaction will run for 15-30 seconds as it sends out more sparks and becomes a bigger green pile.
5. At the end of the reaction, show the students how big the green pile is compared to the original orange mound. Explain that the green flakes are Chromium (III) Oxide.
6. Alert them that if you collected all the green flakes and weighed them, they would weigh less than the orange salt! If you have a white board, draw out the reaction to show where the missing mass went.

Why This Works

Ammonium Dichromate is a strong Oxidizer, meaning that it readily accepts electrons from other sources around it. It is also thermodynamically unstable, and will begin to decompose if it is heated.

Thermal Decomposition

(NH4)2Cr2O7 (s)	→	Cr2O3 (s) + N2 (g) + 4 H2O (g)
Ammonium Dichromate		Chromium (III) Oxide

When heated, it breaks down into chromium (III) oxide, nitrogen gas and water vapor. The nitrogen and water leave the salt, which means the green flakes will have less mass to them! It is worth noting that this reaction happens without the addition of oxygen from the air. Since it doesn't use oxygen, and since it doesn't produce carbon dioxide, this reaction would not be considered a combustion reaction. Instead, this would be called a Thermal Decomposition, since the salt is the only reactant, and it only needs a heat source in order to decompose.

Additional Information

• The ammonium (III) oxide produced is something that we are familiar with, but as a paint or dye. It is used as the pigment for chrome green and for making viridian!
• This demonstration pairs well with the Elephant Toothpaste demonstration
• This demonstration is a part of the Chemistry Show