If a reaction releases energy and heat, it is called exothermic reaction. The opposite is the endothermic reaction, when energy and heat is absorbed.
Thermal decomposition of some materials is quite fascinating and dynamic, so they can be used in the Chain Reaction as well. Let's see an example.
Warm ammonium dichromate with a burning rod. If it begins to sparkle, you don’t need to warm it any more. Warming results in a self-sustaining exothermic reaction accompanied with intensive sparkling which is a very similar phenomenon to a volcanic eruption. The originally orange-red crystalline substance becomes a dark green, loose, powder-like matter.
Tip: Check what formation and reaction heat mean!