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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?

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    $\begingroup$ 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 $\endgroup$ – Poutnik Jan 24 at 15:55
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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 "non additive" volume means.

I don't have the numbers at hand, but the "contraction" is pretty much measurable and in particular important in the mass production of desinfectants and drinks.

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  • $\begingroup$ I'm very sorry, I made a typo, I meant "70% alcohol" in the question. $\endgroup$ – Kaaviya U Jan 24 at 14:25
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    $\begingroup$ You are free to edit your question to make it self-consistent. BTW my answer holds for both isopropyl and ethyl alcohol. You just can't trivially make one from the another. $\endgroup$ – fraxinus Jan 24 at 14:28

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