Unit of measurement for the proportion of space of a substance in the total volume of a mixture. If you mix 100 liters ethanol with 100 liters of water, we obtain 192.75 liters of mixture; This phenomenon is called volume contraction, Since 100 liters of alcohol are now contained in this 192.75 liter mixture, 100 liters of mixture contain 51.8 liters of alcohol. Thus, the alcohol content of the mixture is 51.8% by volume. Another unit of measure is the mass fraction (weight percent). The 100 liters of water weigh 100 kilograms, the 100 liters of alcohol but only 79.45 kilograms. The weight of the mixture is thus 179.45 kilograms. In this case, the alcohol content is 44.2% by weight (instead of 51.8% by volume). The alcohol therefore has a relatively low density.
However, since the remaining components of the wine usually have a value of 1.0 kg / l (water) or higher, the difference to the mass-related component is considerable. It should also be noted that the volume of liquids changes with temperature. It follows that the statement of the percentages by weight is clearer than the percentages by volume. In the case of alcoholic beverages, the alcohol content is always given in volume percent measured at 20 ° C. At the label this is declared in the form of "alc.", "existing alcohol" or "present alcohol" in whole or half percentages, for example, with "alc. 12.5% vol. This corresponds to 12.5 ml of alcohol in 100 ml (or 1 dl) of this wine. See under alcohol content and relativ density, and also units,