why are volumes not additive?

Answer 1

Although we can't see them, liquid molecules have tiny holes (spaces) in them here and there (which most of us like to call interstices) and each of the liquids, solute and solvent, can fit into those spaces. Solvents and solutes whose interstices don't allow the other one to fit into, because of size considerations, are additive but many liquids are not additive. A common example is ethanol and water. I THINK I remember, but I'm not positive, that 500 mL ethanol + 500 mL water, mixed thoroughly together, has a total volume of 950 mL and not 1000 mL.