Why can't we use 30 mL of water and 70 mL of alcohol to make a 70% (v/v) solution of Isopropyl Alcohol?
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.