Can a chemical compound be called a reactant in a dissociation reaction?

Question

For instance, can 2SO3 be the reactant in the dissociation reaction

2SO3 = 2 SO2 + O2?

Answer 1

Yes but the reaction you give as an example probably will not happen in the real world.

H2SO3 >SO2 + H2O
Sulfurous acid disassociates to Sulfur Dioxide and Water.

Actually the reaction in aqueous solution is a bit more complex but the one I cite is Chemistry 101

Answer 2

Yes, anything on the left hand side of the arrow is the reactant. Even though the SO3 isn't reacting with another substance, it is still forming products.