Ammonium dichromate, with the chemical formula (NH4)2Cr2O7, is a chemical compound composed of ammonium ions (NH4+) and dichromate ions (Cr2O7^2-). It is a bright orange-red crystalline solid that is highly soluble in water.

Ammonium dichromate is an interesting compound due to its unique and visually appealing properties, but it is also a highly hazardous chemical and should be handled with extreme caution. Some key properties and uses of ammonium dichromate include:

1. Volcano Demonstration: Ammonium dichromate is often used in educational demonstrations to create a “volcano” effect. When heated, it decomposes into chromium(III) oxide (Cr2O3), nitrogen gas (N2), and water vapor (H2O). This exothermic reaction produces a fountain-like display of sparks and gas, resembling a volcanic eruption.
2. Laboratory Reagent: It is used as a laboratory reagent in certain chemical reactions and experiments.
3. Photography: Historically, ammonium dichromate was used in some photographic processes.

Despite its visually impressive properties, ammonium dichromate is a hazardous chemical due to its toxicity and the presence of highly toxic chromium compounds. When heated or burned, it releases harmful gases, including nitrogen oxides and chromium compounds. Inhalation or ingestion of its dust or vapors can lead to severe health effects.

Due to its extreme toxicity and hazardous nature, the use of ammonium dichromate has been restricted in many countries. It is not recommended for casual or educational demonstrations due to the significant health risks associated with its use. If ammonium dichromate is used for educational or demonstration purposes, it should be done under proper supervision, in a well-ventilated area, and with strict safety measures in place.