Unit of measurement for the proportion of a substance in the total volume of a mixture. If you mix 100 litres of ethanol with 100 litres of water, you get 192.75 litres of mixture; this phenomenon is called volume contraction. Since there are 100 litres of alcohol in this 192.75 litre mixture, 100 litres of mixture contain 51.8 litres of alcohol. Thus, the alcohol content of the mixture is 51.8 percent by volume. Another unit of measurement is the mass percentage (weight percentage). The 100 litres of water weigh 100 kg, but the 100 litres of alcohol only 79.45 kg. The weight of the mixture is therefore 179.45 kg, the alcohol content is 44.2 percent by weight (instead of 51.8 percent by volume). The alcohol therefore has a low relative density.
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