Editing Alcohol by volume (section)

==Volume change==
[[File:Excess Volume Mixture of Ethanol and Water.png|thumb|upright=1.48|Change in volume with increasing alc/vol.]]
Mixing two solutions of alcohol of different strengths usually causes a change in volume. Mixing pure water with a solution less than 24% by mass causes a slight increase in total volume, whereas the mixing of two solutions above 24% causes a decrease in volume.{{efn|See data in the CRC Handbook of Chemistry and Physics, 49th edition, pp. D-151 and D-152. Mixing a solution above 24% with a solution below 24% may cause an increase or a decrease, depending on the details.}} The phenomenon of volume changes due to mixing dissimilar solutions is called "[[partial molar volume]]". Water and ethanol are both [[Chemical polarity|polar]] solvents. When water is added to ethanol, the smaller water molecules are attracted to the ethanol's hydroxyl group, and each molecule alters the polarity field of the other. The attraction allows closer spacing between molecules than is usually found in non-polar mixtures.{{cn|date=October 2024}}

Thus, alc/vol is not the same as [[volume fraction]] expressed as a percentage. Volume fraction, which is widely used in chemistry (commonly denoted as v/v), is defined as the volume of a particular component divided by the sum of all components in the mixture when they are measured separately. For example, to make 100&nbsp;mL of 50% alc/vol ethanol solution, water would be added to 50 mL of ethanol to make up exactly 100 mL. Whereas to make a 50% v/v ethanol solution, 50&nbsp;mL of ethanol and 50&nbsp;mL of water could be mixed but the resulting volume of solution will measure less than 100&nbsp;mL due to the change of volume on mixing, and will contain a higher concentration of ethanol.<ref>{{Cite web|url=https://hbcponline.com/faces/documents/15_20/15_20_0002.xhtml|title=Density ρ of Ethanol-Water Mixtures at the Temperature in °C Indicated by Superscript|website=CRC Handbook of Chemistry and Physics|url-access=subscription|access-date=2019-12-13}}<br>This source gives density data for ethanol:water mixes by %weight ethanol in 5% increments and against temperature including at 25&nbsp;°C, used here. It can be calculated from this table that at 25&nbsp;°C, 45&nbsp;g of ethanol has volume 57.3&nbsp;mL, 55&nbsp;g of water has volume 55.2&nbsp;mL; these sum to 112.5&nbsp;mL. When mixed they have volume 108.6&nbsp;mL.</ref> The difference is not large, with the maximum difference being less than 2.5%, and less than 0.5% difference for concentrations under 20%.{{cn|date=October 2024}}