Consider the reaction.

2S02(g) + O2(g)
2SO3(g)

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged