# 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 "not neccessarily additive" mean?

• You can use 30 + 70 mL to get 70% v/v solution, but the resulting liquid need not have 100 mL, E.g. 1 L of water and 1 L of ethanol give ( after cooling of selfheated mixture) about 1.96 L of 50 % V/V solution. In chemistry, the volume fraction φi is defined as the volume of a constituent Vi divided by the volume of all constituents of the mixture V prior to mixing. en.wikipedia.org/wiki/Volume_fraction – Poutnik Jan 24 at 15:55