Why can't we use 30 mL of water and 70 mL of alcohol to make a 70% (v/v) solution of Isopropyl Alcohol? Volume percentage (V/V): Solutions that are formulated from two liquids are often expressed as volume percent
with respect to the solute. The volume percent is the volume of a liquid in 100 mL of solution. The label on a bottle
of ordinary rubbing alcohol reads “isopropyl alcohol, 70% by volume”. Such a solution could be made by mixing
70 mL of alcohol with water to make a total volume of 100 mL, but we cannot use 30 mL of water, because the
two volumes are not necessarily additive:

What exactly does &quot;not neccessarily additive&quot; mean?
Assuming you are talking about either isopropyl alcohol or ethyl alcohol (ethanol) and not both. They are different beasts, they are both used as disinfectants and solvents, but isopropyl is way more toxic to humans (who more or less regularily drink etanol solutions).
70ml ethanol and 30ml water, whem mixed, do not make 100ml mixture, but somewhat less.
That's what &quot;non additive&quot; volume means.
I don't have the numbers at hand, but the &quot;contraction&quot; is pretty much measurable and in particular important in the mass production of desinfectants and drinks.