Of all the chemical reactions we've covered within the past few months, the one filmed below probaby takes the prize for coolest looking and possibly scariest. After the pile of reactants is lit, it begins to look like tentacles are crawling out of a portal to Hell.
In reality, what's depicted in the video below is actually two reactions: the decomposition of ammonium dichromate and the combustion reaction of mercury (II) thiocyanate.
The orange powder is ammonium dichromate and when heat is introduced, it forms nitrogen gas, water, and ammonium (III) oxide, which is the dark powder that looks like the volcano you see.
What appears to be tentacles is actually what happens when heat is introduced to mercury (II) thiocyanate. The white solid expands when it's heated to become a dark, tentacle-like mass due to its decomposition to carbon nitride. In addition, sulfur dioxide and mercury (II) sulfide are also produced. The reaction is appropriately nicknamed the "Pharoah's Serpent" and was sold in stores as fireworks until people realized it's toxic.