and are the masses additive?
That depends on the substances. If they react and release gasses/use gasses, then the masses and volumes can change. Similarly, different gasses take up different space. The volume isn't constant either.
Masses are always additive. Volumes only sometimes. For example, when you mix water and acetone or water and alcohol, the volume is less than them separately. It has something to do with the way the molecules interact.
