Chemical Concepts Demonstrated: metal/nonmetal chemistry, chemistry of aluminum
The reaction begins and builds up heat. This heat causes the aluminum to melt and float on top of the liquid bromine (which is producing a cloud of vapor because the heat is also boiling it). Wherever the two elements meet, sparks, heat, and light are given off.
Explanation (including important chemical equations):
The reaction between aluminum and bromine is very exothermic (DHo = -1022 kJ). This drives the entire process, and energy is given off in the form of light and heat:
2 Al (s) + 3 Br2 (l) --> 2 AlBr3 (s)
The product of this reaction is a molecular solid with the formula Al2Br6, but the formula is often written as AlBr3.