DiChromate

Ammonium dichromate is the inorganic compound with the formula (NH4)2Cr2O7. This salt is sometimes known as Vesuvian Fire, because of its use in the formation of tabletop "volcanoes". It has been used in pyrotechnics and in the early days of photography.

The volcano demonstration involves igniting a pile of the salt, which initiates the following conversion:
(NH₄)₂Cr₂O₇(s) ? Cr₂O₃(s) + N₂(g) + 4H₂O(g)

Potassium dichromate, K₂Cr₂O₇, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is potentially harmful to health and must be handled and disposed of appropriately. It is a crystalline ionic solid with a vivid red-orange color.

Sodium dichromate is the chemical compound with the formula Na₂Cr₂O₇. Usually, however, the salt is handled as its dihydrate Na₂Cr₂O₇·2H₂O. Its chemistry, appearance, and behaviour are very similar to those of the more widely encountered potassium dichromate. This chemical is around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its equivalent weight is also lower, which is often desirable.[1]