Unit of measure for the space fraction of a substance in the total volume of a mixture. If you mix 100 liters ethanol with 100 liters of water, you get 192.75 liters of mixture; this phenomenon is called volume contraction, Because now in this 192.75 liter mixture 100 liters alcohol are included, 100 liters of mixture contain 51.8 liters of alcohol. The alcohol content of the mixture is thus 51.8% by volume. Another unit of measurement is the mass fraction (percent by weight). The 100 liters of water weigh 100 kg, the 100 liters of alcohol only 79.45 kg. The weight of the mixture is therefore 179.45 kg. In this case the alcohol content is 44.2 percent by weight (instead of 51.8 percent by volume). The alcohol therefore has a relatively low density.
However, since the rest of the wine components usually have a content of 1.0 kg / l (water) or higher, the difference to the mass-related portion is considerable. It should be noted that the volume of liquids changes with temperature. It follows that the percentages by weight are clearer than percentages by volume. In the case of alcoholic beverages, the alcohol content is always given in volume percent measured at 20 ° Celsius. At the label this is declared in the form of "alc.", "existing alcohol" or "existing alcohol content" in whole or half percent, 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,