Aluminum reacts with bromine to form aluminum bromide. What are the reactants ?

Explanation: 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.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.